Most expensive- not really sure, the really pricey stuff I can usually talk someone into getting as a gift... perhaps FF12 CE at launch?
Most valuable- absolutely no clue, as I'm finding lately I really don't remember what all I own... I added .hack GU 3 to my collection list the other day, and as I browsed over it real quick I spotted Tomba and Tomba 2- I vaguely remember buying Tomba 2, but I had no idea I owned the original! (Which makes me pretty stoked, that's a good game.)
Least valuable- any number of crappy liscensed game or the occasional random sports title I get when people I know cut their collections back (can't turn down free games, ya know- sometimes I get good stuff with the junk.)
Or possibly the NES Mario/Duck Hunt cart... my sister's BF often says 'people don't buy this game, they find it alongside the road and take it home- well, that's how I got a copy, anyway.)