If cost is all that is important, you can brew beer very, very cheaply. I just kegged a 5 gallon batch of beer that cost me $5.50!!! It was done as an experiment that I replicate every 4 or 5 years -- brew a light American style lager made all grain that tastes far better than mass produced beers. It also is my test of the "cleanness" of my brewing. If there's any funk going on, I'll be able to taste it in this beer.
The recipe was simple -- 5 lbs American 2 row ($4.25 from MidWest Supplies -- 1/2 of a 10lb bag) and 3/4 oz Cascade boil and 1/4 oz Cascade (finish) ($1.25 -- 1/8th of a $10 8oz purchase made on eBay). Yeast was harvested so it was basically free. Now, there was some shipping on the grain, so if I add $2 (just a guess for this portion of a larger order) it might be a $7.50 batch of highly drinkable beer.
1.032 OG, 1.006 F.G for 3.5% ABV (remarkable good 87% efficiency). It's light and crisp and tasty for a light lager -- which of course isn't my favorite style beer, but has turned out great!
If I really wanted to make a great session beer, I could do it with 7.5lbs grain ($6.50) and an extra oz of Columbus hops (also purchased on eBay, same price -- use Cascade for finishing & dry hopping) ($2.50). Again, harvest yeast, and I've got a beer that's sitting around 1.040 for $9. Add shipping on the grain and it's probably $12.
Mostly I don't do this, though. I brew for fun, enjoyment & challenge and don't worry about the cost a ton. But if you want to go cheap, it is very easy to do.