rentals thru a Rental Company can be as low as $19 per point (non-premium resort/ inside of 7 months). Private owners prices vary, again upon resort & time frame ($13 would likely be points only usable soon, within a very short time frame, when availability is severely limited). In general, $16 - $19 is average for a private rental.
For my brother & SIL, I ask just for the Maintenance fees (they use about 300pts every other year, & are very financially stable - this arrangement is fair to both).
I don't charge anything for our own children (who go at least twice a year), and friends/family on limited income (for occasional trips).
I get the impression it would make your MIL happiest to gift the points to you (as parents do

) - is there any way you could just accept them, and then maybe get her a thank you gift to show your appreciation?