Try looking around on Craigslist a bit and getting an idea... I sold my old crappy 15" "snowflake" style wheels on CL for $150 bucks... I would try starting it out at around 150-200 area, depending on the condition.
I only paid 250 for the steelies that replaced the OEM ones so I only have about 100 bucks in my wheels
I'd be happy if I could $200 for the 4. I can't find much xterra stuff on Craigslist here in Houston or Austin. I'll just throw them up on it and see if anyone bites. Thanks for the suggestions on a starting point.