I've done 69 brews and have kept track of every penny I've spent and every bottle I've bottled (and I when I switched to kegging, I call a corny keg 53 bottles). I focus on bottles because the most intuitive comparison for me to make is how much I spend per 6 pack at the store vs. how much a 6 pack costs me to brew myself. Now to clarify, saving money by doing this is not remotely a motivating factor (It would be way too much work if I looked at it as work!). It's just kinda fun to keep track.
As people have said, the cost of brewing can vary quite a bit based on how you do it, so here are a few facts that should give a sense of where I fall along that spectrum:
-I do not buy in bulk even though I could save $ in doing so. The only reason for that is to support my LHBS.
-Do 5 gallon batches
-Have two chest freezers for fermenting
-Have a 4-tap keezer
-Re-use yeast about half the time
-Brew on a turkey fryer type burner in a bayou classic 10 gallon pot. Mash in a 10 gallon cooler.
Right now I'm at $9.88 per six pack. If I spend, say, $30 on my next brew, that'll go down to $9.77...but of course will go up next time I need some tubing or something. That number has dipped as low as $8.50 or so.
In any case, a few years ago I figured out I spent an average of $11.50 per six pack for commercial beer. Based on that, I reckon I've saved $894 over the years.
Hmm...let me take it to the next level...if I figure an average of 7 hours per brew (that includes not just the brew day itself, but all the ancillary activities), that's 69 x 7 = 483 hours spent brewing. Hey, I'm making $1.85 every hour I engage in homebrewing activities!
Yeah, I'm kidding about that last part. And I'm sure if I were to keep track of how much I've given away to friends and family, I'd be in the negative. But even so, I can certainly say it's an inexpensive hobby.