well small animals can cost just as much as herps. I know a Guine pigs require ALOT of space, Have to have their "toys" and "Hides" in the middle of the cage so they can make "laps" around it!
pet rats require atleast the MIN of 1.5 cubic ft per rat (it is preferable that 2 cubic ft per rat) so a 24"x30"x24" would be fine for 2 rats.......They eat meat and veggies, and like to have ALOT of time out of the cage.
You shouldn't use pine, cedar or saw dust with them becuase they have sensitive noses.
Here is a WONDERFUL pet rat website!
http://www.fancy-rat.co.uk/home/
It's a UK website but they have TONS of great info and they have a cage calculator on there.