Remember that the words "Cancun Resort" and "All-Inclusive" are not always the same. A resort means that you get special amenities that are not included with normal hotels stays. These include things like spa treatments, golf courses, and water sport equipment.
A resort may or may not be all-inclusive. Check the fine-print on your Cancun resort and you may find that the prices for the things to do will outweigh your budget. While the price of an all-inclusive may seem high, factor in all of your costs you would normally spend (in some cases, including tipping) and you'll see how quickly the cost benefit switches over to the all-inclusive side.