Tanks are a big "No-No" in the world of ratties. Not enough ventilation can lead to URI's, which will end up costing you money at a vet's office. Even with a tank topper, there is just not enough air circulation to have a happy rattie.
If money is an issue (and that is why you are looking to add on to your tank), you should check your local Craigslist for a second hand cage. You can find some good stuff off of there, just check in the "pets" section or do a search for "cage".