Last March we paid $40 - $45 for a 4-hr cart rental, it was the last cart available except for another outfit that wanted $90 for 4 hrs (supply & demand). Normally we would pay that price for a full-day rental. You’d need at least 3 carts so taxis would make more sense for your group.
Carts are fun to drive around for 2 - 4 ppl for sightseeing, and you may want to do that one day...
The IM forum on TA is pretty active and you will find more current feedback there. The Top Questions section is also helpful...
https://www.tripadvisor.com/ShowForum-g150810-i360-Isla_Mujeres_Yucatan_Peninsula.html
