My condolences on the loss of your friend.
Pro Topline were good heads back in the day, and would still be able to hold their own relatively well today. The company was based out of New Zealand.
RHS (Racing Head Service), which is part of the Competition Cams Group (aka Comp Cams) bought Pro Topline a number of years ago. Looks like they've completely dropped the cast iron heads and now only produce the aluminum ones.
I found an old website on the Way Back Machine archive with some info:
https://web.archive.org/web/20110317...htsteelsb.html
I have a catalogue with a bit more information.
As for what you could price them out today: well, the price of aluminum heads has come down so much that a pair of brand new bare aluminum heads, with about 20 years advancement in technology, go for less than $500 USD.
I think your best buyer would be either:
1. nostalgia engine guy who ran them back in the day and wants another set
2. racer that, by class rules, must run a 'stock style' iron head that doesn't have to be stock / OEM
3. performance person that prefers iron heads to aluminum
I wish you good luck with the sale! Realistically, though, I wouldn't expect to retire from the profits.