If you're looking for something in the Pi price range, a Beaglebone Black seems about right and the TI SoC it uses has tons of publicly available, detailed documentation. On the other hand, if you want more features/periperals/cores, and don't mind not-so-thorough documentation, one of the boards based on Chinese SoCs could be more fun...

Here's a huge list of others: http://socialcompare.com/en/comparison/low-cost-arm-boards