Depends, could be rarity that dictates the price, it should be in most cases but it's usually what hot at the moment with the dancers and DJs. Soon as something starts getting played dealers push up the price and "DJs" will pay that bit more, rather than find their own records and maybe the next biggie
Take Ace Spectrum. It was a record you couldn't give away at one time, it sold in it's millions. All of a sudden for some strange reason it gets a few spins and it's suddenly it has a three figure price tag, f*** knows why. It's back down to £25 nowadays due to demand dropping but even that is 5 times it's value IMHO. It's a record that should never have been expensive regardless of dance floor status.
There's records that are in the £50 bracket that are harder to find than some records that cost several 100 quid and more!!! SWONS!