Personally, I'd rather send the $20 to you/the EAA project fundraiser in exchange for having 40 (60?) of those pages scanned into the HBA archive.
Requested donation rate per page (50 cents per page? A dollar?)
The problem is that (assuming that the same number of HBA people participate in either scheme) if I used your math, it works out to far less money in the build project.
If there are in fact exactly1,000 desirable decent quality drawings in this box, and let's say HBA members will actually want half of them enough to release the moths from their wallet, that's 500 sell-able drawings.
With my original scheme, that creates $10K in gross revenue, minus envelopes and stamps.
With your first idea ($20 for 40 drawings scanned) it would result in $250 in gross revenue.
With your second idea ($.050-$1.00 per scanned page) it would result in between $250 and $500 gross revenue.