It costs 157 A Gemstones to apply any SA to an A grade weapon (except of course for dual swords).
Sorry, but this is wrong.
For example, each of the 3 SAs on the Sword of Miracles costs 140 A gems. The cost varies depending on the weapon and, in some cases, the SA. Until someone is able to compile a list for all the weapons, the only "convenient" way to find out how many gems you need is to go talk to the Blacksmith of Mammon while you have your weapon in your inventory.