The Eagle2 was a (successful) project to build a $50 satellite (cost of hardware, not including launch). It fits in a 50mm cube, transmits at 1kbps at 437.505 MHz (Doppler shift can change this by +/- 10kHz), and uses 100mW of transmit power (for reference an iPhone uses ~2W). This is pretty remarkable when you consider that this satellite is in low-earth orbit, over 1000 miles away. Of course, there are no trees or buildings to get in the way either. Data is sent using FSK RTTY. It can also receive messages from a ground station (these need to be sent at higher power so the satellite can receive them with its small antennas). Lots more details at the link above. I would not have thought a functional satellite on this budget was possible until learning about this project.