Polishing a rocker cover isn't hard. So I'd say around the $50-$100. But keep in mind that you can only get what people are willing to pay. I mean, there are a dime a dozen of the other "Datsun OHC" rocker covers around. You're one is special because its the 68 model with 1600 on top. IMO suits someone who wants it for the nostalgia of it... Others my think otherwise however.
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Ongoing Project: 1972 Datsun 510 S13 SR20Det.
http://ozdat.com/forum/viewtopic.php?f=33&t=17898
New Daily: D22 Navara (The new workhorse)
Retired 12/2016: MY98 Subaru Impreza RX
Previous Car: Restored Green 1972 Datsun 510, Hot L18