Used Prices...Honestly, I don't really know. While I am about to type out a bunch of info, I am surely not a expert on H&G.
Some recent history. H&G was sold to Ballisti-Cast, the son attempted to take over Ballisti-Cast from the Dad and failed, the Son then went to Northern Valley Machining and started a sister company NV Ballistics. I don't think NV Ballistics exist anymore, but Northern Valley Machine does.
New Prices:
A couple years ago, NV Ballistics was making some molds to order and also sold Handles...the Handles were $37, I believe the molds were around $200 ea., but I think NV Ballistics customer base was mostly for Ballisti-Cast casting machines or the Magma Master Caster.
H&G molds are collectable, I would think minty ones like these two marked "Geo A Hensley", that date from 1933-1941, might be more collectable, but as you know, collectability is tied to desirability of the cavity design...as well as rarity.
that link is one part of a larger website.
The Hensely & Gibbs Bullet Mould List & Reference Website
www.hensleygibbs.com
With that said, I'm not a collector, at least I try not to be, I am a bullet caster. As a bullet caster, I figure a minty vintage H&G 2 cav Iron mold should be worth no less than $50.
Honestly, I bought these two H&G molds and two Handles, specifically for the one cavity that's 401, but I made an offer on everything left in that particular lot, to get a good price, they were listed a bit higher. At some point in time, if they don't make special bullets to my guns liking, and I get tired of playing with them, I'd probably list them for $100 per mold/handle set...unless I discover they are worth a lot more? Again, I honestly don't really know the values. They are rare enough, they don't turn up on my usual searches.
The $20 I paid for the two Modern Bond molds is likely a bust, but it'll be a fun project, I may or may not succeed