Thanks for cross referencing Don. While I got your attention, do you have any suggestions for a cheap place to stay between Tulum and Playa del carmen when we go across to dive the cenotes?
Well, I posted the link to your other thread showing that you booked at Blue Angel, but people are still suggesting Coz hotels.
I don't know the hotels on the mainland well. You may want to ask on the main Mexico forum. Google bought Kayak.com I think, but I still like shopping there. Since you ask for PDC-Tulum area, I ran a search in January for hotels within 25 miles of Akumal - which causes some confusion as the results included Cozumel too.

I still arranged results by price and looked over the list, but that didn't work.
Next I opened separate tabs on Kayak to run searches on Akumal, Tulum, and PDC respectively...
Not many choices in the $100/night range in Akumal, I think that is a ritzy area, but there are a couple to consider. It's only 40 miles of good hiway from Tulum to PDC tho so let's look at others.
Tulum has lots of hotels reasonably priced, one 3 star listed at $26! Look over the descriptions closely tho as I have been disappointed in a some 3 stars (filed complaints to change ratings - grr!

) but I think you can find a nice one for less than $100, even 3 to a room. Arranging 3 beds in a room will always be a case by case challenge.*
PDC has even more hotels to pick from, and 4 star hotels starting at $95. You'd still be close to many cenotes and less than an hour from Tulum. Kayak does list the Cielo which I hated once - but too late to complain about their rating. Lots of 3 star hotels starting at $57, and I like the location of the La Matatena Hotel, close to ferry dock.
You may well want to decide who you are diving with over there, then shop for hotels close to them. Kayak works well usually.
* Getting 3 divers in one room still boggles my mind. I just don't like to be that crowded! I'd rather spend more and get 2 connecting rooms if possible, share the space & bathroonms, and split the costs.