It isn't technically a microprocessor, but the Roboard RB-100 is a full computer for around the cost of a NXT kit (250 alone, 300 with everything). It has 24 pwm/servo io channels that can be regonfigured in software as gpio. It also has 3 usb ports. It isn't very well equipped as far as memory, but the CPU runs at 1GHz.
http://www.robotshop.com/roboard-starter-kit-3.html
I'm not sure giving anything autonomous the ability to fire even airsoft pellets is the best idea -->
, although it would be cool for laser tag.