Macae Brazil is cheapest at like 33/night. In fact the area is right on the water, but not sure what there is to do there. It's like 100 miles north of Rio.

Two mexico cities are under $40/night. Heck you can get the park Hyatt in Chennai for 47/night with breakfast.

Really depends on your level of flexibility (and of course what you're comfortable with/where we're allowed to go during Covid) .

Would be fun for a group to all go to one place as a flyer talk mattress run.