It's not so much that PVC is expensive but that finding 1/2 inch thick at a hardware store is difficult. I know the 1/4 inch thick is about $48 per sheet of 4'x8' but I am not sure the 1/2 inch stuff. I guess the problem is to build a rack with it it might cost nearly as much as from one of the vendors so yea, You might save $50 even $100 but think of the time you are going to spend and if you don't have the right tools then it for sure gets more expensive. Add in that you won't be able to get the cutouts for like probes and heat tape unless you have a router (which gets even more expensive) and you end up spending a lot
Idea: instead of using acrylic paint, why not look into deck sealants? They have those paints that keep a deck weatherproof so I don't see why it wouldn't work for a project like this. Once they dry, they would be non toxic to the snakes.