Cage Questions

I currently have a cage that IS big enough for my two baby girls... but I'm sure that in a few months when they're bigger that won't be the case. What's the best idea.

I'm a poor college student with little money to spend. Would it be cheaper to buy a huge cage for like $80, or should I instead try to build my own cage? Has anyone here built their own cage? How did you do it?

OR, are there cheap cages for sale anywhere?

Thanks for the help!