FREX, the DM might like to drop in a +1 Sword of Frost Fire-type Bane because it fits the NPC and might make an interesting/useful backup weapon in the long haul. But the PC is probably thinking: "Sweet! I can cash that out for 9000 gp and I can afford to upgrade my gauntlets from +2 to +4."
Seems like a pretty extreme example to me, but then, I've never seen someone say that.
Because they're so [bleep]ing expensive! That's exactly my point. It's absurd to say "people get stuff which is useful, regardless of price". People get stuff which is useful, sure. But useful is a function of (among other things) price.
Okay, maybe I exaggerated a bit. But value!= price. Value is a function of price and
utility. If the rings of wizardry cost half what they do now, they
might be more valuable, but the simple fact of the matter is, the CLs for those things are way too high, as Merric pointed out - by the time you can make a ring of wizardry IV, you're casting 9th level spells, and a couple extra 4th level spells just aren't going to be as useful at that level (certainly not worth 100K gp). If the CLs were dropped by, say, 5-7 levels, then they might be more valuable.
As for buying things no matter the price: how many times have you seen people save up money to buy something that's really expensive, but that's either really cool or something they really need/want? If they've got a horde of less- (or not) useful items, they're going to dump those off too, getting the best price they can for them, most times, so they can get more money to buy the one big really cool thing. I'm not saying it happens all the time - it doesn't. But it does happen.