I have had that problem.
What I discovered, is you can make passable large rat cages for very little money.
Go to a hardware store, or someplace that will sell large plastic containers.
They are definately large enough for rats, cut a large hole in the lid and cover it with chicken wire (be careful to attatch it on the outside so the rats dont harm themselves on the sharp edges).
Dont worry they are super cheap! They only cost about $20 and thats for the huge ones.
If you are worried about ventilation, just cut some holes on the sides and cover them with chicken wire too.
For the chicken wire use a staple gun or something.
With mine I have even got pvc piping and made tunnels between them so they can run from one to the other... its easy to make... go to a plumming store and usually they will give you as many offcuts as you need.
You can use these for rat toys too.
Hope I helped