DIY cages and Zoo Med cages are made in the same factory in China... they are made of materials that rust. There have been posts on the forum about this. I think one of the threads has photos of the person's cage with holes in the side from the rusting screen.
The cages sold at LLL Reptile.com are made of stainless steal screws and aluminum screen, these won't rust and last for years and well worth the money. I only have cages purchased from LLL.
1XL - Had for a year+, no rust.
2L - One I bought new, no rust. One I got used, rusting (older cage).
4M - One new, no rust. 3 Used, all starting to rust.
The older cages I find are definitely poorer quality, newer cages they made seems to be holding up significantly better.
I have one of the newer ones and I have no rust on the screen although I DO have rust on the hinges. I've had it for about 3-4 months now and I carry it outside 3-4 times a week. Though I can't say much for the strength, the screen has dents in it and sometimes the door doesn't close correctly.