Breezer, first of all, look at the evaluation from a qualitative point of view, not subjective. That means, take any emotion or passion out of the equation. Then give yourself a honest aprasial of what skills you have, tools, space, and other resources (buddies with tools and know how). Then the evaluation becomes very simple:
MV - P - OSW = PV
Whereas, MV is the Market Value of a car fully restored to your expectations that you can buy and drive today, P is the Parts you will have to replace, new or used, or cost of restoring the part, and OSW is any Out Sourced Work that you will need to pay someone else such as paint, engine/trans/diff rebuilding, etc. Needless the say, the more skills you have the lower this number. I do not include my time as I actually enjoy restoring cars and hunting for parts. So that is a wash. And you do not have to account for everything, just a ball park. So you found a very nice drivable Elan for $25K that you believe is market price. Then you found a project. Project needs a frame, so that's $4K, rebuilding the engine is about $5K, interior dash, dash pad, seat upholstrery, top, say another $5K. Mechanicals such as clutch, mounts, busings, another $1K. Paint if you job it out is $8K. So based on these ball parks, the car is worth $2K assuming you didn't miss anything, and I am sure you did. Now don't focus on the numbers I used, rather the philosophy and it is important you understand it is selective to you and your capibilities. Someone else that can do alot of the expensive OSW work themself has a much lower cost bases and could offer more.
Now, with that said, I also look at it from the other side. What is the project car worth in parts in a fast sale? The same project car has a shot frame, value zero. Seized motor and drive line, $2K, Body $1K, other bits and usable parts, say $1K. So now we have $4K, but it all has to be disassembled, cleaned, sold, and shipped, so deduct $1K. PITA job just to break even, but it gives you a gauge if you had to cut losses and dump it. So you are now between $2-3K would be the strike price.
I have learned a long time ago, you cannot buy a project car cheap enough. I have walked away from cars given to me free, as it wasn't worth the effort, and I'm one of those guys who has the capibilities to do everything. Check out my site at
http://www.TheLolaRegistry.com under project cars and see some of the nonsense I get myself into. Restoring these cars has been a 40 year passion for me, and while I cannot honestly say I have not spent more than market value in restoring all my cars, sometimes, you just have to say No. Good luck Allan