The cost is around £30 per paper (depending on board). If you're looking for a single mark I'd send both papers off so you're looking at around sixty pounds. If your grade changes you will get a refund. The increase in grade boundaries is irrelevant, if the examiner has made a mistake and missed a mark your marks will go up and your grade will change. Assuming of course that you don't lose marks elsewhere. If money isn't tight I'd always recommend a re-mark if you're one mark off. Having said that I've sent a fair few off and as yet none have come back with any changes. Good luck.