The C500s are one of the more common clones but they are still sought after. I'd expect to get between at least $150 for it but $1,000 is high. That sort of price is reserved for the ultra-rare clones like the Daystar Genesis MP or SuperMac S900DP/S910.
If it's a J710 it's a much more unusual machine but they are not in crazily high demand. I think you'd see a sale price of at least $250 for a J710 but it could go a lot higher if it has one of the unobtainium G3 upgrades.
Ha. Yeah they weren't terribly pretty, were they? Akia in Japan made a rebadged version of this with a slightly different front panel and a stand to orient it on its side instead of as a desktop.
Anyway UMAX were one of the more interesting clone builders in that these are Alchemy-based systems but they use an entirely custom logic board (Apple's Alchemy systems were the drawer-style logic boards for the 54/6400s) whereas lower-tier cloners just used a variation of the Tanzania/II architecture (which were mostly just rebadged StarMaxes). I think UMAX and PowerComputing are the only ones who built custom Alchemy systems (and UMAX would sublicense these designs to others such as Akia).
I doubt I'd pay more than $100 for it unless it was one of the top-end models with the fast L2 cache on the processor card, or one of the elusive Akia or Tatung or other off-brand cloner's version of it. I think I paid something around $100 for the UMAX Apus 2000 (which is the same thing only sold outside of the States) that I bought about two years ago.