Unfortunately, this is high season and the prices are outrageous. To stay at a 3 star hotel with the best beach in Cancun is $1800+ per person for a week. However, if you wait until April (a week after Easter) the prices drop drastically. The weather here still isn't all that great and it's beautiful down there. Around that time, you're looking at around $1300-1400 for a 4.5 star hotel with a nice beach.
Places I recommend checking out:
Gran Caribe - 4.5 Star hotel, decent beach, amazing food.
The Royal - GT Luxury resort (better than 5 star), anything you could ever dream of you can order to eat, amazing rooms, nice beach, great pool
Kin Ha - 3 or 3.5 star, nicest beach in Cancun, decent food, nice people. It's a smaller condo complex type of hotel, not a huge luxury resort.
Whatever you do, stay away from the lagoon side of Cancun, prices are much cheaper but the beaches suck.
I can ask my dad for more suggestions, he used to run a tour company down in Cancun and has been there probably 20+ times.