This topic gets a lot of controversy becasue it all depends on what you want the car for.
If you are buying a nice cruizer to take to an occaisional local small show, documentation, to me, is a non issue. How is the car built today? How does it look and run as it sits in the driveway? Which would you rather have, a perfectly rebuilt 68 SS with a new crate ZZ4 motor, rebuiilt trans , new interior, with nice paint and no documentation or an all original 68SS with the original 350 and the car is is in good condition with decent paint...but it has build sheet, window sticker, and POP? Are you going to restore the car for the show circuit, then go for the latter. Are you going to take it out in the evenings and weekends for a fun day of cruizing?? Then opt for the former.
One caviat to that above is , generally speaking, if the car is in equal condition, then documentation will add some value, just because the market drives it.
As far as cost, for lets say a #2 condition 68 Small Block SS car in my neck of the woods you can figure 15K-20K (U.S.) If it has some documentation and matching numbers motor, maybe add 2-5K depending on the car and the documentation. These are very rough numbers but generally, this is what I see in the Northeast Ohio area.
My 68 RS Z/28 came with no documentation but the dyno sheets for the blueprinted 302 motor that is in it. I cana't even prove it left the factory as a Z/28. I do know that if it was cloned, it was done pretty correct because it has all the Z/28 goodies and some things that were added like a 4 wheel disk brakes. I did not care about the fact it had no documentation becasue the car was just beautiful as it sat in the garage and it ran just as good. Now it is mine...no documentation and all. I suppose that when the day comes to sell it my market (people willing to buy) won't be quite as big because I don't have the documentation. But you know what, I don't care because it will be their loss...not mine.
Sorry to ramble...the world according to 'Da Mouse
R. Stacho (a.k.a Mytmouse @ Team Chevelle)