I have seen them, as I'm sure you have, listed "buy it now" for $1,000-$1,500 on ebay. I've watched a few of those, and it seems they get relisted again and again and again, so they must not be moving quick at that price.
I've heard of a few guys that have traded them outright for a stock/core Dana 60, so figure those are worth $600-$900 tops.
I have heard/seen that they seem to sell better as a pair with the rear. I've seen a couple auctions for a pair hit around 2K. It also helps greatly if you have a way to ship (to a more affluent area)
So....front alone, local sale, this economy, I'd think around $600-700 would be a fair guess. Personally I wouldn't pay that much, maybe $400-$500.