The price should be based before but because the price is so high I would just talk to the DM in a home game/campaign which can be a problem in Pathfinder Society. The reason that the cost is so screwed up is that chain shirt is either 6350 gp (after) or 12600 gp (before) if you determine by weight whereas adamantine is only 5100 gp and gets lots of cool perks including dr 1/- which is a much better deal so the 500 gp/lb. was probably meant for adventuring gear like locks which weigh very little. I think the price for mithril weapons should be like 100 gp/lb. before determining weight but should add masterwork (300 gp). So a mithril greatsword should cost 1150 gp, 50 (base) + 800 (8 lbs.) + 300 (masterwork) rather than 4050 gp, 50 (base) + 4000 (8 lbs.). But even that makes chain shirt 2900 gp, so that way isn't perfect either. In conclusion Paizo should just release specific rules on mithril weapons, but for now if your characters are spending that much gold on melee then weight shouldn't be a problem.
