To be honest with you Mark, I am not concerned really with the difference between the Grey sheet and the PCGS price guide. I know there is a difference with some coins. What that difference is, I don't know? When I price coins, I usually use either the Grey Sheet (if one is avaiable) or the Coin Market Guide by Coin World. So when buying higher graded UNC coins (ie, anything above MS 64 typically) I try to pay between MS60 and MS62 pricing. This being because I cannot tell the difference between the two grades, so if I can't, Im not going to pay it. Maybe a flawed way of thinking, but it works for me. This way, I feel better with my purchase and I am sure the seller is making some money too. If the coin is raw, I very rarely pay above MS60 or 61 prices. This is what I stick with and it works for me. Does that make sense?
It makes sense to do what you are comfortable with and enjoy, hopefully on an informed basis. Just for fun, I am going to take 2 real life examples and illustrate them here. For the record, I don't know the answers before I look them up... Very recently, I bought an NGC MS67 Norfolk out of auction for $517. Checking "graysheet"......bid is $500 and ask is $550. And the PCGS Price Guide price is......$850 I also bought a really pretty (though my images don't capture the color well) PCGS MS66 Roanoke for $460. "Graysheet" bid is $310 and ask is $340, but the PCGS Price Guide says.....$475. I think you can quickly see that there are large % price differences between the two price sources and that they shouldn't be treated as one and the same. Here are images of the two coins in question: