Raspberry Pi is not the only cheap educational computer. When going to lower end on the performance there is Arduino, but it lacks display output and network (if you add those to it it gets easily more expensive than Raspberry Pi).
FIGnition Rève describes itself as the definitive £20 educational DIY computer! It works like an 8-bit home Micro: outputting to composite video and ready to be interactively programmed from the moment you switch it on. The Rève now has bitmapped graphics; sound and audio saving/loading as well as 8Kb of RAM; 384Kb of storage; an 8-key keypad and runs a variant of FIG-Forth. It uses USB for power; firmware upgrades and program downloads. I saw FIGnition first mentioned in Raspberry Pi: the modder’s dream machine? article.