If you build a pen out of anything other than otherwise garbage materials and a small roll of the cheapest fence, you are going to be spending even more money.

Also whats with people buying like a dozen chickens? Do you eat an entire dozen eggs every single day? No? Then you don't need a dozen chickens. 2 chickens will often result in people giving away tons of eggs because they have too many. Maybe a few years down the line when they lay a few less eggs you can add another one or two. If you don't eat 90%+ of their eggs, you will once again be losing money.

Also unless they are free roaming over a very large area, you do not want any roosters. Roosters in a small coop and/or yard often get aggressive and they will attack you. Yes you can cow them down if you are quick enough to grab them, sometimes mid-attack, but most people aren't because they don't want to get stabbed with their spurs. Also buying sexed chicks are not a 100% guarantee you won't get a rooster, ive gotten multiple roosters out of sexed chickens and often the only right choice is to kill them because you don't want a bunch of roosters fighting either each other or attacking people.

Chickens are social animals and require a pack. A dozen seems excessive though.