I think the 5906 is a kind of "sleeper" among modern pistols--it's a very good gun, but it doesn't command a lot of attention. Anyway, $500 is high. The hi-caps make the price more tolerable, but IMHO it's the gun not the mags that should set the selling price.
BTW, my dealer--who used to be reasonable but whose prices have risen dramatically in the last year--would probably charge $450-475 for the 5906 with two hi-caps, and sell the third hi-cap for $60 extra.
The bottom line, of course, is that the gun is worth what anyone is willing to pay.