Balance of prices on the world market would be key. The NPC market would have to buy low and sell high. The PCs would make money inside those ranges, or by filling some other niche deficiency designed into the system, such as quantity, delivery speed, range, etc.
The NPC market would have to sell based on what it costs now to build a thing in order for it not to be punishing to people who don't participate in the new wood/stone/etc economy, so that there would ultimately be no change in price.
I do think that any new resource being introduced is a new way to get gold though, so perhaps there should also be a drop in tax revenue corresponding to what you'd make just selling the resources as vendor trash to the NPC, then within that metric you'd have people selling it for twice that and people buying it at what would be bargain prices, and everyone makes out a little bit better.
