I have a sort of formula. Note, I am not an "expert" by any means.
However in 2013 I bought a 1.5 unrestored (Brittle) Golden Age key, and I recently purchased an additional copy of the same book, Restored 7.5 (extensive resto, but not TRIMMED)
I'm not sure which book counts as the "undercopy"
Note.. my thoughts do no apply to SLIGHT restoration. E.g a "Slight" resto 7.0 of a GA key, where the resto could possibly be reversed... obviously that's a special situation! And I wouldn't bid on a book like that b/c I wouldn't be confident (don't know enough about resto and how to reverse it)
So my own "formula" applies to more like , R-Extensive 5's, 6s and 7s.
I believe there is value in those books especially for GA mega keys... for instance Tec 31, not enough copies to go around. I would gladly acquire a nice Restored Tec 31!
In my point of view, the 'grade' of the Resto just doesn't matter much. I don't value an extensive restored 7.5 much more than a 6.0. Maybe 5-10% more or something. That's just me.
ASSUMING the resto book is not trimmed, I believe it has to be roughly valued to an unrestored 1.0 book? Or possibly a 1.5.
If it's restored and TRIMMED, then I might value it at 50% the value of a 1.0 unrestored book.
Other aspects of the restoration might matter more, or less (TRIMMING is the big one, for me).
But my point is, for an extensively restored book, I'm valuing it at 0.5x to 1.5x (roughly) of the same book in a 1.0 unrestored condition.
I don't think "30% to 50% of the same grade in unrestored" holds much water (not with me, anyway)... and isn't what I've seen in the auctions lately for major GA books.
Again, this is all just my PERSONAL formula and thoughts. I don't claim my formula tracks the market or other peoples thoughts on pricing. On the other hand, I do have skin in the game and "voted" with my hard earned dollars!