"The cost of strengthening and silvering of a weapon is for extra materials. Silvering only takes 15pp, so 15pp plus 150cp equals 165, so charge 3gp 3sp to silver a weapon. For strengthening a weapon is 50pp plus 500cp equals 550, so you would charge 5gp 5sp."
(OOG: do you own a ARB? It's all on page 58)