The high numbered commons can be picked up in lots for less tha $5 a piece. There are exceptions of course. The Rookie Norm Cash usually cost more than you would expect., Maybe Low grade T206 money. There are also the AllStars and the Corsair Trio (Clemente). The biggest dawgs will run $50+. The Hi #'s are very do-able, not very hard to track down.
The error cards are up to you. The Spahn card has a premium for sure but I don't care about most of the others.
Grey vs. White? I agree that White is more pleasing but I didn't bother with both.
