VEX is aimed at highschoolers but can be very east with some help. VEX gives students designing, building, programming, and of course bugfixing, and it is in your budget range depending on how many kits you want to buy.
if you feel it is too expensive or out of your budget range
Lego NXTs have programming and building but design is a bit less emphasized on those.
good old lego robots don't seem too complicated or fun but something like
http://www.ecs.umass.edu/ece/labs/codes/lego/ can always make them complicated