Hi,
I am by no means an expert on this subject but I did remember reading something online a few years back about this very situation. The following text is from the now defunct Scuderia Topolino website forum.
"I get a lot of questions from people all over the world as to what a good price is for a resto-project, and of course there no easy answer. I will say that as a general guideline, if you take what the value of the car will be when you are finished and take 25-33% of that, then this will be a good ballpark number. Of course if the car has load of pedigree and race history, then the price will be higher. As an example, wonderfully restored ex-Roosevelt Bialbero team car sold in Monterey for over $300,000 dollars. I remember when this car was bought prior to restoration, and the price paid was right in line with my "guideline". At the moment I have a very nice Allemano Coupe for sale on my website. The car is complete with the drivetrain, all glass, upholstery, seats, door trims, instruments etc and the asking price is USD$25K. Considering that a restored example recently sold for US$71,000, again the formula seems to work."
Hope this helps.
Great find, I'm jealous!
Cheers,
Toby