Re-mark prices differ slightly for each exam board, but I seem to remember them being about £35-40 last year.
Remember that you are only refunded if your module grade changes, not for changes in UMS marks. So even if you were moved up 5 UMS marks, but your grade did not change, you would still have to pay.
And I'm sure you know this but remember marks can go down as well as up. So requesting a re-mark just to try move up one mark to the next grade boundary might not be the best idea in the greater scheme of things, as you could make up that mark in the next modules and risk loosing marks with a re-mark.