Should we offer a cash incentive for our son to do well in his A-levels?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in next Saturday's paper. This week's question My husband wants to offer our 17-year-old a £1,500 incentive to do well in his A-levels. He lacks motivation at the moment and I suspect it will work as he's very money driven. But it feels wrong to me – what do others think? Does the end result justify the means? What are you thoughts?