There is a universal changer coming from Toshiba but their track record thus far is pretty poor this year. Their hd upscaling player was recalled. They've been known to put out a player or 2 with the chroma bug too..
It's listed at $179. I can't imagine the audio quality being that good though...
If you want SACD and can go w/o dvd-audio then Yamaha has a faroudja based changer for $399. I'm not sure how good it is but it looked rather good at the store.
If your talking about a 5-disc changer then there are only about 2 or 3 out there, I know Yamaha has one, I can't remember the others. However, the $500 cap will lead you nowhere, I think you asking for a little much. I went through this same dilemma a few months ago and all the universal players have more then a few problems with one format or the other, although the Yamaha tested the best overall, and it's certainly not under $500.
I decided to go for the next best thing and get a mega changer (400 disc) that will do everything but DVD-A, and I'm glad I did. This unit (Sony DVP-CX777ES) with it's RS232 port can be controlled by a third party software program from you computer that gives you vast benefits over any of the other changers on the market.
I think Pioneer makes a couple of HTIB's that have an integrated 5 disc uni player. I don't think this is what your looking for or should even be seriously considered (lousy amp section) but technically, it's a uni changer.