What's the best affordable resort in Riviera Maya?

The NOW resorts (there are two of them; Now Sapphire and Now Jade) might be a good fit, and within your budget. I was at the Now Jade last month. It's a pretty nice resort. (I'd say it's a solid 4-star) It's in Peurto Moreles, which is about 20-30 minutes south of the Cancun Airport.