In my experience, costs on projects like this are loosey-goosey because
A) The space is typically donated. "Do you have anywhere we could park an XP-82 for a few years?" "Fuck yeah I own an aircraft hangar and this is my kind of tax writeoff!"
B) The labor is typically donated. "Hey semi-retired machinist how would you like to spend your evenings and weekends restoring a rare XP-82? There's cola and popcorn."
C) The parts are typically donated, or vastly underreported. "Hey grandma - would you like us to haul that Merlin intake manifold out of your barn where it can find new life again or would you like to go through the hassle of trying to sell it only to discover that the only customer is us anyway?"
Which is not to say none of this stuff has value, nor is it to say that there isn't a formidable amount of expertise and effort expended. Generally, however, projects like this start from the viewpoint of "obviously we can't afford to pay what this is worth but won't it be fun" and continue through the viewpoint of "fuck yeah this is fun." It takes a village and nobody restoring warbirds does it without a whole buncha folx donating time money parts and energy.