I bought 2 used Metal Detectors off of eBay last year, right after Christmas - many people are in need of cash and will sell stuff cheaper then (including those on Craigslist). The dealers pretty much stick to what they agreed to sell the products for. If I buy from a local Dealer (better than some large online sellers), they usually give a better package deal (along with other benifits).
A free BOGO detector (from some online dealers) is typically is worth what you paid for it - it might sound like a good deal, but unless its really something you want, it may not be valuable to you at all.