Warning: Picture heavy.
Here's some of my past cages. Most pictures were taken right after cage cleaning and all their toys weren't dry yet. For the few who hated toys -- all my rats have/still do get out time for the majority of the night.
Collapsible, $20 ~$40. Not for girl rats! Small girls can squeeze out. Chinchilla cage by superpet.
Coast Cages -- $70. I love this cage. It is by far the easiest to clean, the most durable, overall the nicest cage I've ever had. Comfortably accommodates the needs of 2 large boys. May be able to fit 4 smaller rats (dwarf, female). I never planning on having girls again so if you try it let me know how it works for you.
Got this cage for free from someone who wanted to get rid of the sweetest chinchilla I'd ever met. It appears to be a homemade cage. Galvanized coating, not as good as powder coating, but better than nothing. If you find one like this I'd say go for it. Only trouble is it's very heavy, and bulky thus making it hard to clean.
This one makes a wonderful quarantine/sickie/old rattie cage. It also fits on most desks =D. Also, if you notice the background.. yogies are a great bonding material lol. So stock up when buying new rats! 24" cage (size of a 20 gallon) made by All Living Things. Around $30.
I can't seem to find a front-view of this cage... so this pic does it no justice. Here's a warning for everyone; I impulse bought the following cage for $80. The bars were NOT coated. The pee soaked right in, so after a few months the cage reeked of rat urine. The urine coated the corners of the cage from top to bottom and the only thing that got the cage clean was hours of scrubbing with small metal brushes. I bought this cage with my girl, Mikiboo, who is still alive. She's 2 years 7 months. I stopped using this cage a year ago -- the smell was horrendous. So take my advice and don't impulse buy non-coated cages.
But no matter what size cage you buy, there will be times where it's never big enough for all of your rats.
Hope this helps everyone in their decision making =D. If you want my opinion, and you're not keeping more than 4 rats -- go with Coast Cages. They make WONDERFUL cages! Super Pet, Martins Cages, and Ferret Nation are also good cage makers.