well if ur budget is 200$ then i would refer to my own post...maybe you could get some usefull information from there http://www.societyofrobots.com/robotforum/index.php?topic=2573.0
i have made allmost a whole project in theoty there
from the schematicks to CAD and ofcourse i couldn't do it without this amazing forum...
anyway what i was saying is that if you would make the same robot that is on the CAD i made, then you could make the robot do several different things like wall following, wall avoiding, if you add some photoresistors to it you could make a light seeker robot, if you add some IR emmitters and recievers then a line following...and all that just without changing the programme code
so it's like manyrobots-in-1
(if you know what i mean