I haven't sat down and figured it out but I would say that my wheel has payed for itself, considering that most of the wool I spin costs $8-15 a pound and exotics are usually much cheaper to spin than buy as yarn. If you want to compare the price of the fiber to the price of handspun yarn, and not millspun, then the equipment will pay itself off even quicker. Also, if you're willing to dye, a pound of white wool roving can make any weight or color of yarn in any (say 2 oz of dk weight green, 4 oz of laceweight blue, 10 oz of bulky hand dyed) amount while a 100g skein of say, worsted weight green wool yarn can only make something that calls for green worsted weight wool yarn, and if you only needed 3 oz of it you now have an extra half ounce or so that will either get thrown out or stay in your stash unused.
I'm not counting the fact that I now have a stash of fiber much larger than my stash of yarn though
I also must admit to still buying yarn.
If you're concerned with cost, there is drop spindling. Sure, it's slower than the wheel, but it's alot cheaper to start out with, and any skills learned on it can be translated over to the spinning wheel if (when) you decide to get one. I spin alot on drop spindles, and though it is much slower, it's also transportable, so I can take one with me in a bag and spin when walking places, when waiting, whenever I have free time, and the free time adds up.
I must remind you of the hated question: if you can buy a sweater/hat/pair of socks/etc... in a store for cheaper than the cost of materials, why knit it? the answer is pretty much the same for whether or not it's economical to spin or not. The process is a big part of spinning and if you like it, then it's another hobby and it's worth it.
One more thing to keep in mind: the resale value of spinning wheels and some other fiber prep tools is very high, so if you don't like spinning you won't lose much of your investment.