"For cheap"
You may want to look into used caging. People are in and out of this hobby frequently and you could find a very good deal looking at classified adds.
As mentioned, the above cages are good options you could also look at Boaphile, and Constrictors NW makes Pro-Line cages.
I have Pro-Line cages and they are single door, front opening cages.
I will house our royal in the top cage 48" x 24 x 14" when the coastal carpet moves into a larger enclosure. It's more than adequate for a 4+ foot royal.
I think you have a good chance of finding a used plastic cage on E-Bay or maybe a member here is selling something.
If you go with Animal Plastics, buy a decent sized cage with a divider and you could add another snake!!