I've rented to friends as well as via the rent/trade board. For strangers, it's the going rate. For a friend, it's $10 - it's easy to calculate for everyone and it keeps both parties on even ground - no one has to feel like they're doing the other a favor or feel like they're obliged to return a kindness later. And it's a great deal of the rack rate (not so much compared to free dining).
As others have said - make sure your friend knows that, if they wanted to cancel, maybe they could not. This is the stickiest part when renting to friends, it would be very difficult if someone could not travel due to sickness or death in the family - if you would loose your points, do you tell them they can't get a refund because "those are the rules" or eat the cost yourself? Depends on how good a friend.