I got Brain Training for the Nintendo DS and it is wicked, really excercises the brain, like arithmetic, reaction speed and other cognative stuff, i'm not scientist like tigersam258, but I know excercises like that every day or every other day are definatley good for helping you work out problems and equations better, and the way you see your Brain Age getting younger (20 is the best) is cool becuase it motivates you to get better!
Worth it in my opinion if your daily activities don't involve much brain work, your brain is a muscle and it needs excercise for optimal operation.
As for prices, check out google products search aswell usually turns up alright but as tiger suggested, kelkoo, dooyoo, ciao etc are good comparison for prods... don't forget second hand from your nearest GAME, Blockbuster, Computer Exchange (CEX) and the infamous EBAY.CO.UK!!