Cheapest ... I picked up one for $125
Most Costly ... I see them on craigslist for $400-$500
Unlike many of White's other machines, the DFX is a competent multi-frequency machine. The MF-VLF technology is essential for ocean beach hunters that want to venture in the wet sand and can be equally beneficial to relic hunters that hunt areas with heavy mineral deposits. With that said, it is not waterproof and compared to new technology, the DFX is starting to show its age. It's far heavier than an Equinox. Its processing speed is relatively slow and you have to sweep it at a significantly slower pace than your CTX.
In the used market, its my opinion, the DFX compares to Minelab Explorers and Quattros (Old version of the Safari). Now there are a lot of variations to the Explorers. The S and XS models are selling for $150-300 and the II, Se, and SePro models are $200-450.
The price is whatever it is worth for you to have a backup/spare unit. Personally, I feel its about a $175-$225 machine. If it is in showroom condition with the original box, manuals, etc., I may go slightly higher. NOTE: Remember to check out the battery compartment and look for corrosion!
If you review the listing and sold prices on eBay you will probably see units in the $350-400 range; however, the seller also is paying 10% of that in fees eBay, 3-4% fees to PayPal and some sellers also include free shipping which deducts another $30 or so.
Again, its a great backup beach machine but its not waterproof.