I would say that a lot will depend upon whether you are trying to price to "unload" or if you are trying to help sell off the estate of a friend. Then the next would be condition. And some styles are more desirable than others, but that might just be a personal preference..
Two equal molds, I would choose the one that has not had the cavities marked for orientation or to segregate the bullets. Others would prefer to know which bullets came from which cavity. I am a casual shooter, the serious shooter has a different set of priorities. Lyman has made at least four sets of handles over the years. Single cavity (small) two cavity (large) Nut cracker (hinge on the end) and a newer style that hinges like the first two. Condition is a factor and while handles can be replaced, a set might bring more if the wood is tight and not charred. Some bullet styles are discontinued and would be favored over a similar new style.
I suppose a common guide would be about half the price of new and comparable molds available from vendors such as Midway or MidSouth. Better condition might bump up the price. Molds not listed in current catalogs might be more desirable for specific rare calibers. $12.00 for a good iron single cavity might be a little on the low price. Have you checked the buy sell and trade section of this forum. You might want to follow the prices there and then see how the prices for molds are running.