Soldato
Price is the point at which your desire to have them is greater than the desire to retain the money. For me, £50 is the line. If you think anything in the world is "cost plus" then you are misunderstanding the world.
I mean that savings does not count in terms of discount, i.e. you spent £50, you did not save £40, if the full price was £90.
Maybe you didn't mean that because it was a secondary point.
As for the credit card, i stoozed a long time ago, been there done that. My credit card balances are £0 right now.