Not much - I bought a tank in Graphitan 5 years ago it had a slight kink in its front leading edge that was invisibly repaired prior to repainting. cost me £100 delivered.
With any damage it's not worth a great deal from a graze to a dint it still needs repairs and paint which doesn't come cheap . You would be better off wait and paying more for a good tank you can just bolt on unless you can get it repaired on the cheap.
Tony
Bought a 'mullered' R1150GS tank complete with fuel cap and all internals/connectors etc ready to plug and play just cos i wanted the filler cap assembly for my GSA.
Turned out the silver tank was in much better nick than i thought but had more dents then Jimmy Savilles reputation so i had in sprayed up in SE colours along with a spare beak i had lying around. Made a nice 'lighter' and more nimble 22 litre tanked GSA.
Paid £50 for the tank but that was when the R1150GSA was the 'current' GS so around 2005