Microcontroller: Arduino Uno from Amazon for about 6-9 dollars or Arduino Nano for about 7$
Web connection: Ethernet Shield on Amazon for roughly 8$
Solar: Small solar panel on Amazon for about 8$ or so.
(This is roughly the cheapest you could get these)
Total Cost: Around $23-25
You could use an ESP8266 (ESP-01) as processor and WiFi as connection (if possible) for 1.50€.
I don't think $50 is too unrealistic, but a lot of the cost depends on whether what you are doing is serious or frivolous.
If you are running on solar you might find a few nasty surprises if you just hook a Uno to a cheapo solar panel, indeed it is very likely. You need to look into low power operations, and choose your components carefully. I don't know about the Nano but it is almost certainly a better bet. You also need to look at battery backup, and charging of same. You may find that you are better off not using an Arduino at all, but getting an ESP8266 module instead, as it is specifically designed to do the sort of job you have in mind. It costs about $5 and I believe it has very low power consumption.
One important piece of the puzzle nobody has mentioned is distance - it sounds like (with the solar) it is some sort of stand alone gadget monitoring something and it needs to connect to the "web" - what kind of distance are we talking about here? Is this something up on a mountain or something somewhere that needs to communicate across several miles or is it something 50 feet away from the nearest wifi access ?
Thanks a lot guys for the reply. The ESP8266 would be ideal, and this is the first time i've heard of it, completely surprised at the cost, but the problem is im looking to somehow do this without a Wifi connection... and I'm unsure how this would work and assume this is where it gets very expensive. (i.e over the $50 or $100 range)
@GPSMikey. You have it right, stand alone gadget monitoring something and needs to be connected to the web. It won't be up on a mountain but in a city. The closest wifi access would be miles away.... so I don't think that's a possibility unfortunately. Is it possible to move the data via SMS?
Consider this, there will be many of these devices scattered throughout an area, miles away from the other if that helps in anyway. Is it possible to relay data from one device to another, in sort of a chain fashion by radio... like XBee.... until it reaches one final point that has wifi access and can upload this data? This is me just talking out of my ass here, I assume what I just said is completely impossible.
But yes, the additional information I need to add is that there is really no wifi access nearby, however there would be multiple of these devices scattered throughout the city if that can somehow be leveraged through radio relaying of some sort.
Cheers guys. I appreciate it.