My wheel refurber just goes on size, no extra for diamond cut from them.
Seem to remember its about £125 for an 18".
Don't think you were robbed at the price you paid.
I quite like what you've done actually - think it looks interesting - its OEM but not. Tasteful.
I might have gone with all black if it was me, but maybe only if the car was a light colour. On the other hand, the silver won't look filthy the second you drive away, which black will.