That's kind of a big question. What size? With or without cabinets? Which cabinets? The "colour" makes a huge difference in price with the "reds" being by far the most expensive generally. 15"s of any type are at least double the price of the 12"s, and the 12"s nearly double those of the 10"s. I got a pair of 12" Golds in Stuart cabinets (cabinets in decent useable shape, but not showpieces by any means), insides and drivers/crossovers looked near brand new, concecutive serial #'s, hangtags, for $1,000 on a local pickup. I felt that was a really good deal, but not a total steal. I would have expected to pay around $1000 plus shipping for the pair of drivers/crossovers alone on ebay and that would have been an "ok" price.
By the way you don't ever want to buy these without crossovers, unless you're really getting them at a steal. You'll end up paying nearly half again for them what you paid for the speaker if you try and get them seperately on ebay.