Should mods to a vintage amp make the price higher or lower?

There's an ad on my local Kijiji for a '79 Marshall JMP, modded by Mike Soldano. Most original JMP heads seem to be going for around $1000 - $2000 CAD, and the guy sellling the modded JMP has no asking price, and just wants an offer. Approximately how much should I offer for this amp head? Thanks in advance!