Did you buy that lathe, or were you thinking of buying it? Rivett's are kind of rare and were well made, so you see people asking pretty high prices for them at times. It's difficult to give a solid answer regarding price because they don't come up often enough. I might be wrong, but my sense of the Rivett market is that there a small number of Rivett fanatics out there that all think they're worth a fortune. Personally, I wouldn't pay more than few hundred for it, but that's just me.
Do you want a fully functional lathe to make parts on, or are you more interested because it's a cool old lathe?