What's a reasonable holiday rent to charge family and friends?

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 Saturday's paper. This week's question We have a static caravan near a beach, used 10 weeks a year. It costs us £3,200 a year upkeep and we'd like to rent it out to friends. We're not looking to make money, but don't like the idea of people holidaying at our expense. What's a reasonable rent to charge – should rarely-seen family pay less than good friends?