Does anybody know of a cheap chassis that has tank treads?
I would actually suggest going for separate Tamiya components: You can purchase the tracks, baseplate, and motors separately. The kit that was linked does not use the dual-motor gearbox, which will be needed if you want to do anything more than move forwards or backwards. Be sure to also pick up the replacement 6V motors for the dual-motor kit (as the standard 3V motors are kinda weak).
Another option is to look for cheap toy R/C (and wired remote control) tanks; they aren't as easy to find, but you may find something in a thrift store or on Ebay. Once you remove the shell and other parts, you'll be left with a very nice platform.
EDIT: Something like this one:
EDIT: Another option:
Dagu Rover 5? I have 2 of them: My first one, have now fitted mecanum wheels: http://www.bajdi.com/?attachment_id=625 And my latest project, has an ATmega2560 and Raspberry Pi with camera: http://www.bajdi.com/dagu-service-droid-now-with-raspberry-pi/dagu-service-droid-front-2/
It's a cheap platform, there are different versions with 2 or 4 motors and with or without encoders.
Just an old R/C tank chassis would do. http://www.amazon.co.uk/RADIO-CONTROLLED-LEOPARD-AIRSOFT-BATTLE/dp/B003JN1YHI/ref=sr_1_8?ie=UTF8&qid=1392038518&sr=8-8&keywords=remote+control+battle+tanks