We don't pay for good grades - we just never have. Then again, my kids are homeschooled so we don't DO grades throughout the year, only on the final exams.
But we DO reward/punish based upon performance - so I agree with the incentive ideas everyone else has posted.
Upon advanced completion of a subject - for example, my son will be completing Algebra II next week, nearly a full semester before he was expected to - if he gets an A on his final exam he will get a reward. The reward varies - sometimes it has been a pre-paid card for paintball, new gear for his fire Explorer program (that stuff gets expensive) or concert tickets. This time he wants a new PS2 game - fortunately the big sales are hitting for the holidays, so the timing on this was good. lol
HOWEVER... for LAZY work and completion after the expected/scheduled goal date, the kids lose things.... priviledges, use of game systems, TV, etc. - whatever seems to fit the crime. My daughter dragged her feet on Science last year and finished the book - and still only getting a C on her final - about 3 weeks after the end of the school year. She lost TV, PS2 and phone priviledges as she completed the subject and for an extra month afterwards. The reason? Those were the distractions that kept her from completing her work in the first place.