Arduino Wireless COMM Help

Hey guys!
I am relatively new to Arduino, and with this project I am about to talk about, I'm learning more things as I go, so please (if you reply) explain things in a noob-friendly way. Thanks!

I am looking for a way to have multiple (not a fixed number) Arduinos communicate together with a Central (or master) Arduino that receives information (4 numbers) from different 'slave' Arduinos. I would prefer it if the master can identify which 'slave' it is receiving info from. I also need a module (bluetooth, RF, etc.) that can do that. The 'master' Arduino will not be sending information back (one-way communication). Is there a Module that can help me achieve that? I don't particularly want to buy 300 dollars worth of xbees, so maybe a cheaper alternative I can use. The distance does not matter too greatly, I just need to it stretch across a high school workshop (10-20 meters?).
If the number of machines does matter, we are dealing with 5 to 15 machines.

There are many modules that can do that.

The cheapest 315MHz/433MHz modules can do that, but they are slow.
They are about 2kbps, with protocol overhead, and with 15 machines, I guess you can transmit 4 bytes every 10 sconds (rough estimate, I could be wrong).

How much data do you want to transmit ?

This is the buying guide from Sparkfun:
I think the "General RF" section is the first to look at, and "Nordic" the second.

When you need 20 meters in a workshop (with lot's of metal machines, people walking around, mains electricity, and so on), you better look for a 50 meters or 100 meters range module.

I'm only looking for something that is capable of sending a 2 digit number when the 'slave' computer detects a signal (not necessary in this topic). The module should be able to send a new signal every half second to five minutes. I hope this is useful information.
I haven't yet read that spark fun page due to limited time.

EDIT: How would I go at the sketch part of things? Does anyone have any guides on this sort of stuff?

Well... read that page.

There are libraries for the RF modules. You have to find a reliable library.

The very cheap 315MHz/433MHz have no software 'interface', they are switched on-and-off. The on-and-off switching (ASK) is done with a library. The best library for that is RadioHead:
But twice a second with a number devices is too much for these cheap one. The range is 20 meters in bad conditions and even less with a bad antenna, so you need a better module anyway.
Better module means spending more money. Sorry, there is no other way.

So you need a better module, one of the RFM or RF22 or CC1100/CC1101 or so.
That RadioHead library supports some of them. The RadioHead page has a list with a some of links where to buy them, so you can check the price.

Some cost a lot more than a simple RF-Bee (which is a CC1101).

You can buy modules on Ebay, they are cheap and with some luck they are also good. Start to search for CC1100 and CC1101.