I had the same thought as the OP, as I'm sure a lot of us did once. Granted I got into this for the experience, but I was curious about how long it would take to break even. My usual beer is roughly $17 a 12'er, so that's $68 per 48 beers (not including tax and redemption). I figure I'm in for about $400 in equipment (and another $300 will be added to convert a mini-fridge to a kegerator), and an average of $38 per kit. So according to my quick "guestimation" math, I'm saving $30 per batch, versus what it would cost to buy the same amount in the store. Meaning I should come out ahead after 23-25 batches. Of course this is assuming that I don't upgrade here, swap pieces out there, etc....
I'm currently working on establishing a pace to give me a new finished batch every two weeks, once my pipeline is running at it's optimum. So even accounting for things like adding another primary down the road to increase my pipeline output (or brew some longer beers without sacrificing my projected pace). I figure at best...as my setup stands now.... I hit a break even point in just over a year, at worst in about a year and a half.
Now, with all that being said, this cost analysis was something I really only gave thought to after seeing what kits were priced at, on average. My main reason for starting this hobby was to be able to enjoy beer I made with my own bare hands. As well as having some styles that aren't readily available without taking a 1/2 hour drive to the nearest local beer & wine superstore. Not to mention trying to concoct my own recipes once I get more knowledgeable about the process as a whole.