the STM based boards like the STM32F discovery board are currently about $12USD
Yes they are cheap, I have a couple myself. However the tool chain in a pain and it takes a long time to get all the drivers linked in. The examples don't work on many tool chains. It took me most of a day to get the blinking LED working, and two days to get the voice recorder example working.
I was using a trial version of Cross Works on my Mac. When the trial ran out it thought it was not worth the £100 or so for the software.