I've never played a Monette trumpet or anything, so I obviously can't say anything about how they play, or the craftsmanship and what not put into his instruments...however: I think that dcbrown1 has some good points. Dave Monette has a lot of good things going for his instruments that help drive those prices up.
1) He has multiple "top players" that have and do use his products. To name a few, Wynton, Maynard, Charles Schlueter, Art Farmer (and TONS of others) He also did some stuff with Doc back in the day. Having these top players playing his instruments are just like certain sports stars using certain brands of golf clubs, baseball bats etc....it brings in customers!
2) He's built up a major mystique with his instruments. It makes trumpeters believe there is something special and different (magical) about them. Maybe there is and maybe there isn't??? It's amazing how easy it is to sneak into the human brain and make people believe things...just by slipping certain words into a sentance. One phrase I tend to here in various Monette videos is "double high C" or "much easier high range". Those phrases tend to get a trumpeters attention!
3) He's a MASTER business man. He knows his craft, and he knows his customers. The man knows what he's doing, and he knows how to make a profit.
4) His instruments are unique. He offers different "custom options" that many companies don't. Integral mouthpieces, his decorated instruments, the high dollar finger buttons, etc...
Well, those are some of my thoughts on the whole Monette discussion.