What about the Matum? Not within walking distance but just a short cab ride away. But still can't beat the Platino for easy access to PUCMM.
I toured this hotel today. It by no means is a Platino, Los Jardines or Century Plaza but is conveniently located, clean, safe, no frills and very inexpensive. You can use the gym, there is a cafeteria/bar across from the registration area with wifi and ample parking.There is a nice grocery store nearby and the new La Sirena just down the street.Hotel Monumental on Autopista Duarte. Newly-renovated and about $25 per night. Each additional person is $10. I have not yet seen it, however, several people we know have stayed there without major complaints. It is nice enough and VERY near PUCMM, less than a 5 minute walk.