Half of the cost is airfare. We pay a couple hundred bucks for a rental car from Easy Way, and split a 2 bedroom condo with two or three other people for a week. Tank rental and fills has become really inexpensive, since we do one long cave dive a day (so one set of doubles and a stage). We eat breakfast at the condo and eat lunch in a restaurant, and then snack and go out for margaritas at night.
If this would be your first MX trip, though, I'd HIGHLY recommend hiring a guide, at least for some of the days. Although some places, like Dos Ojos and Grand Cenote, are easy to get to and easy to dive, there are other caverns that would are not typically used for the tourist dives, which are very beautiful and would be great for a certified cavern diver. They aren't always easy to find, and sometimes you just need to know who to pay or where to get a key.