Other people can tell you their buff value matched against a price, but its very hard to find a real template for item buffs.
Cos you see other people might add\substract items from the default inventory, some might want to have a forum with weak weapons, some with strong...
I myself do not know what the best weapon buff values would be, so cant help ya there sorry
On a general note, Vivi of this thread and SquareUltima forums made a Vertical RPG Stats addon, due to appear in the Addons post in a matter of seconds.
Since its HTML and not made by me, it is outta my league. Hence, All Your Support Whining Are Belong To Vivi