Just found a 68 Formula S 383 Cuda. What's it worth?

I think it's too high for a non-running "as found" A-body, even though it is pretty much the most desirable package you could realistically find (not likely to find an SS/A car). Just add $20k and see where that puts you -- that would be your minimum cost -- you could easily spend more. For this model, you could stretch to maybe $8k. IMO.