I think the best thing, by far, would be the 'custom' part of the purchase experience. I've never had an instrument custom-made for me, but I have to admit that it would be nice to get it tailored to my idiosyncratic needs and taste. But you can get that from several world-class builders for far less than $25k. So the extra is for cost-no-object materials, and prestige value.
I've never wanted a Lamborghini for the same reasons: I can't afford one. I don't drive nearly well enough to appreciate one. I don't have to get anywhere that fast (even if it was legal). And I don't want something that screams "Hey! Steal me!" to the world. Would I love to try driving one sometime? Absolutely, but I'd never own it, even if I could afford to.
Same for a $25k bass.