In fact, the prices you pay for a pre-booked travel package to Cancun, hotels included, are often ar

Don't assume that hotels will be as cheap in Cancun as Mexico hotels generally.  While there are classic expedia travel seller largest dirt-cheap Mexican hotels in Cancun, in general, expedia travel seller largest advertised prices are not as dirt cheap as their counterparts in, say, Mexico City or Acapulco.
But most North American travelers to Cancun don't worry about that.  By the time most North Americans come to Cancun or Mexico hotels have already been booked for them as part of their package tours.  If you are considering a travel package to Cancun and/or other Mexico destinations, a package that includes hotels is probably going to be cheaper on the whole, at least if you only want to consider mid-range and higher hotels with amenities such as air conditioning and room service, and can avoid opting for expensive expedia travel seller largest extras like room service once you arrive (as the hotel owners expedia travel seller largest are counting on you to do).
In fact, the prices you pay for a pre-booked travel package to Cancun, hotels included, are often artificially low. Basically, hotels, resorts, restaurants, car rental agencies, airlines, and the like band together to get into a travel package, each offering prices that are often below cost, in the hope that you'll buy expensive extras. expedia travel seller largest  The hotel prices you can get in these kinds of package are even more dirt-cheap in recent years thanks to the cutthroat competition among hotel and resort owners; more than a few Cancun hotels are not currently turning a profit expedia travel seller largest at all.  Their owners' goal is to corner the market and establish lasting relationships with tour operators by offering incredibly cheap deals.
Thus, you're unlikely expedia travel seller largest to find any better deals if you book once you arrive in Cancun or Mexico; hotels will only sell to you at their advertised expedia travel seller largest prices, unless you buy into a travel package after you arrive expedia travel seller largest (but then, you've missed out on the chance to get reduced airfare). expedia travel seller largest The exception expedia travel seller largest to this is if you're looking to stay in one of the two new youth hostels or small, older hotels in the downtown area.  Not only are these hotels even cheaper than the hotel price incorporated into package tours.  At these hotels, you're less likely to be gouged for extras such as room service, including things you might not have realized were "extras," such as fresh fruit to go with your complimentary continental breakfast of instant coffee and a packaged roll.  Note, however, that even these less popular options should be booked expedia travel seller largest in advance if possible.