I would figure out what the point difference was between a one-bedroom and two-bedroom villa and then calculate at at $10/point. For example, a one-bedroom villa for one week during May would be 225 points and 285 for a two-bedroom. The difference would be 60 points, or $600.
If you would otherwise get a studio, the points would be 112 points for the same week, or a difference of 173 points, or $1730 for the week.
If you want, average the point differences and you come out with 116 points, or $1165.
So if you want to be nice, let them pay the $600. If you want to be reasonable, let them pay $1100 (they could not get that accomodation at that price). If you want to lose them as friends, charge them $1730.