Here's an interesting article comparing the costs of various mopping options.
Personally, I love the ease and convenience of the Swiffer wetjet, but I don't love the cost of buying disposable pads or new bottles of cleaning solution. Since I mop almost every day, it'd cost me a fortune to stay stocked in pads and solution.
So, I simply refill the bottle myself (Instructions here and here) with a mixture of vinegar, water and rubbing alcohol (so it dries faster) and use reusable, washable pads. I have a couple of pads that a friend crocheted for it, and if those are dirty, then I fold up a microfiber cloth and use that. It sticks to the velcro on the Wetjet just fine.
I don't like using an old fashioned sponge mop because they can't be cleaned and that bugs me. Besides, most of our floors are laminate and sponge mops are just too wet for laminate flooring.
According to the article, the Swiffer Wetjet is the cheapest of the quick mops to purchase, but more expensive eventually because of the price of the refill pads and solution bottles. Other quick mops that include reusable pads and refillable bottles are more expensive to purchase but cheaper in the long run.
I figure because I made my cheaper-to-purchase Swiffer into a cheaper-to-maintain resuable/refillable quick mop, I'm coming out ahead of the game!
Ultimately, an old-fashioned mop and bucket is going to be the cheapest route. But for me, sometimes spending a little bit more for peace of mind in cleanliness is the better choice. :-)