Every one is going to say "invest in the CN! its worth it!"
BUT if you can't do that, then I'd say the super pet exotics cage, but its really only big enough for 3 males. Its what I've got, and its kind of a pain to clean, but my boys like it, and its got the levels. I don't know much about the martins cages, but a lot of people really like them.
I wouldn't advise making your own. A lot of people here get all excited and make one, and then wish they'd re-directed the funds for something else.
Also you can look on Kiji or Craigslist for good deals. Thats where I got my Super Pet, and I only paid 40. Right now in my area there is a double FN, already wire meshed, for 150. I want it really bad, but I have no money. Plus then i'd end up with 80 rats lol, and my roomies would hate me.