Here is my take. It is strictly my opinion, yet based on seeing many many examples sell on
ebay. I will give a varied answer as not all restored coins are created equal by any means.
For strictly acid-date coins;
poor detail and major color distraction with residual chemicals still eating away at the coin; basically useless
.....you'd be surprised at how many people just restore coins so very badly
poor detail or major color issues, but not both; about 10% of G-4 value
decent detail and well taken care of; about 20% of G-4
For full coin restorations;
if coin surface and detail is fairly compromised; about 15% - 25% of G-4
if coin surface and detail is good; anywhere from 30% - 75% of G-4
For hybrid restorations;
if coin shows off color and/or weak detail; about 20% of G-4
if coin looks almost exactly like original anywhere from 30% - 75% of G-4.
For really good hybrid or full-coin restorations; sometimes buyers will even pay much higher, occasionally exceeding G-4 or even VG-8 original prices, but this may be due more to those buyers not realizing the coin has been restored as it looks strong and "original" enough. Also, so many buyers never read item descriptions that say the coin has been restored.