From what I've seen over the years, the box bottoms usually sell for a couple dollars a piece, depending on the player and condition. The Gretkzy cards usually book higher than all the others. In hockey, when comparing OPC to Topps, generally speaking, the OPCs seem to go for a small premium. For example, if the OPC Gretzky sells at $10, I would expect to see the Topps version going for $8 to $9. Not a huge drop in price, but a small one.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 10/25/2023). I have added a large portion of my inventory to the site, and currently have trading turned off (details are in my profile). I have (finally) unearthed my 2 very large boxes of Star Company minor league sets and they are available (email for details).