Today I found a build of RF transmitter using one mosfet (and AVR). So, I started thinking. Say, we take the smallest solar panel we can find, like 22x7mm KXOB22-01X8-ND from DigiKey. Or CPC1822 in SO-8 package from Sparkfun. Then we add 3$ 0.4mm thick PCB from itead. On the other side, lets mount the most radiation resistant and low power MCU we can find - something from Texas Instruments MSP430F* FRAM series. The rest of the board space will be used for RF tranciver and tantalium capacitor. What we just got is a fully functional satellite with BOM cost less than 10$. If using SO-8 package the size of the entire satellite will be 6x5x3mm.
Why not start building it? I am still waiting Texas Instruments to come out with MCU that has FRAM and RF built in. FRAM is needed because it eliminates power consumption and makes it radiation hardened. Both necessary for space environment.