I have a micro sized robot and need a radio for it that will allow 1 way communication from a PC or joystick of some sort to the robot. I just need it to send enough information to tell the robot to stop, go forward, go backwards, turn left, and turn right. I am looking for a transmitter that is small and high powered as well as cheap. I don't need much bandwidth a message to the robot about every half second is sufficient.
Why high powered? If its microsized, it can't be too far away and still be able to see it to control it, thus not much power is needed.
or
http://yourduino.com/sunshop2/index.php?l=product_detail&p=188
Get a copy of Forrest M. Mims III's "Engineer's Mini-Notebook: Communications Projects":
In it you will find more than a few simple transmitters and receivers (AM and FM - but FM is more complex) - of the AM transmitters, the simplest narrow-band circuit consists of a resistor, a capacitor, and a hand-wound coil, and can be tuned from 550 KHz to 10 MHz. One which can be used to "transmit temperature or light intensity" (from the book), consists of only a resistor, a potentiometer, a capacitor, and a small signal (2n2222) transistor, plus a hand-wound coil and a small battery, in a "Hartley Oscillator" configuration.
There's a code oscillator transmitter made from a 555 plus some other parts (could drop the Arduino in place of the 555 if you wanted); there's also a voice/sound transmitter shown.
Simplest AM receiver shown uses a hand-wound coil, a capacitor, and a germanium diode (basically a "crystal" receiver - no battery needed if you use a small crystal earphone). More complex receivers are shown using small audio amplifier circuits (a 741 and LM386 combo).
All of this is very low bandwidth, and probably noisy as hell to get working reasonably, but the parts cost will be damn cheap (you could probably throw both a transmitter and receiver together for under $5.00 USD). Transmit/receive distance won't be great, though - but it might just fit your needs, plus they would be extremely low-power systems - and you'll probably learn a ton about RF building them!
Hand building a transmitter/receiver like that and then trying to get digital data thru it is not going to be at all reliable.
Get a simple 433MHz Tx/Rx unit and use Virtualwire to send messages.
Personally, I wouldn't bother fannying about with either a 433mhz system nor with a hand-built rx/tx pair.
Although, I could never read mention of Forest Mims without remembering with fondness the series of
electronics notebooks of his I have. Also, the time I nuked everyone's tv reception in the neighborhood as
a kid because I left a ~100Mhz transmitter running on batteries for a week and a half on the same breadboard
that was flashing an led. I realized after about 10 days that a new and funny band that was appearing on the
screen co-incided with the flash of the led. When I removed the batteries, the reception cleared back up.
The kicker was when a neighbor asked later that day or the next if we'd had any trouble with our tv for
the past week and a half. 
Building that radio was the best electronics fun I had since dad
helped me make a radio-bug that fitted in a matchbox a decade earlier.
Since you've asked for cheapest-radio, I'd just go with an industry standard nrf24L01+ set.
Not only can you get the modules with varying output power, they're 'all' pin-compatible with one-another
and cheaper than chips. You should be able to get a pair of them delivered for less money than
either 1/2 of the 434mhz solution CrossRoads suggested.
You should be able to have them delivered to your door for around $2 ea - $4 in total for two transceivers.
So, they're cheaper and are bidirectional. You could receive messages back from your robot as well as
being able to control it.
If you'd like more range than the 24L01+ chipsets give, you could just get a pair of A7105 modules. I've bought and
had a pair of them delivered for $6. They're more sensitive and have a lower data-rate, which translates into
better range performance.
Also, each of the ones I mention handle ID, pairing and noise rejection in hardware. No need to waste time and effort
re-implementing what can be (and is) done in hardware.
434mhz Tx : US $3.95
434mhz Rx : US $4.95
(link provided earlier in thread)
nRF24L01+ TRx : AUD $1.47 ea (need 2)
http://www.ebay.com.au/itm/1X-NRF24L01-2-4GHz-Antenna-Wireless-Transceiver-Module-Microcontroller-WST-/300951649158
A7105 TRx: AUD $6.57 pr (need 1)
The 24L01 is in all kinds of industrial equipment, while r/c gear made by Hobby King, Husban, FlySky and others all use the a7105 module. There are threads at rcgroups.com on making your own Tx or Rx that's compatible with commercially available
Rx or Txs. Turning on a laser pointer from over 200 meters away was a giggle one night. Having 10 simultaneous 16-bit channels
(65536 values used to represent 0-100%) with either of the transcievers is no more difficult than a single 1 (very easy).
Depending on the output requirements - PWM or simple on/off - It's usually harder to drive the arduino outputs than send the channel info.
Commercial units (FlySky) transmit at intervals as low as 1.5ms, which is great for responsiveness and low-jitter operation.
There's no requirement for you to do so as often, many wireless temperature sensors only transmit a packet every 30 seconds.
How much range do you need? Would IR instead of radio be a possibility?
@enhzflep,
The 2nd transceiver I provided a link for was the nrf24L01+.
Needs SPI interface vs equivalent of software serial that virtualwire uses, so a couple more pins.
Don't know how critical that is to arduinoPi.
433 Tx/Rx pair can be found for less also - this Rx board looks to be a little smaller too.
http://yourduino.com/sunshop2/index.php?l=product_detail&p=341
and some other options
http://www.robotshop.com/productinfo.aspx?pc=RB-Ite-108&lang=en-US
These are not the cheapest, however, you may want to consider them.
http://www.ramseyelectronics.com/site/default.asp?search=433&page=search&x=0&y=0
Optional encoder/decoder models add security and allows multiple devices on the same frequency
Pi,
You say 'micro' bot. So, one would assume that you want the smallest package possible. You'll be able to get a smaller overall package using a roll your own discreet recvr/decoder (google Pericomm Tech), but it is a reall pain in the @&& if you aren't fluent in RF. You could try the 300-400MHz modules mentioned above, but they are mostly AM (very noise suseptable, but you can usually increase your reception by trailing a longer antenna), and you'll also need something to decode and send drive to the motors. You could also try the canned NRF2401 type boards NRF24L01+). They're complete tx/rx, multichannel, hi-freq, noise immune, easy to use transcievers. But they are probablly the biggest (landscape wise) of the solutions, and you also need a microcontroller to finish out the system, on both tx and rx sides. With whatever solution you choose, remenber that you need more batt power if you go bi-directional (more power usage when transmitting), so that's probably out for a microbot. I would still add my vote to the RF2401 modules purely due to the simpliity of use and low price (cann't offer an opinion on Xbee, just started playing with them). There is a guy who spends a lot of time here, sharp as a tack, and has an internet shop (YourDuino.com??) that sells a variety of add-ons for the Arduino (RF2401 boards included), Terry something, seems like a nice guy. you could probablly get a lot of help from him.