We have had chickens for 5 years now, we are just about to hit the reset button with a new flock as they are really starting to slow down in production. We have had up to 10 of them, but are down to 6 since we have lost a few due to what we think was just natural causes. I don't really like the chickens, but my wife does so we keep doing it. During the non-winter months we average about 0.625 eggs/chicken-day, or in other words about 44 eggs per week or about 15 dozen per month when we had 10. Feed cost was about 0.250 lbs/chicken-day or about 75 lbs/month which is about 2 40lb feed bags which are about $16-$18 a bag (nothing fancy). Considering the cost of shavings for the coup, anti-peck lotion, etc., they cost us about $40/month. So we were getting backyard fresh eggs for about $2.67 per dozen. We easily sold excess eggs for $4.00/dozen, so there is a small amount of profits to be had and decently cheap eggs for backyard fresh eggs. Another way to look at it is you could sell 10 dozen and have a free 5 dozen per month.
HOWEVER, there are negatives. Firstly, they are loud. Like, really loud. Be prepared to have them out singing their egg song starting at 5:30 in the morning during the summer... if you don't know what the egg song sounds like, youtube it, it's not a soothing melody... you don't sleep though it, neighbors hate it even more than you do. What we started doing was locking them in the coup at night time and not letting them out until 9:00 or so, that way it stays dark and so they think it's night time. And if they do start singing, it's at least muffled.
Secondly, they stink. Staying up on cleaning their run helps, so do shavings and diacromatious-earth, but they still stink, especially in the summer.
Finally, the eggs take some time and effort to clean (the tend to get covered in crap on occasion), so it's not just gathering the eggs every day... it's a fair amount of work. People often don't realize that. Not a huge deal, just to be noted.
Not sure what else to say, but just wanted to lay out the math and the pro's / con's.