As Mark eluded to the cost savings at the DIY level are nowhere near as great when working with plastics.
The biggest reason to do it yourself is to achieve a dimension you particuarly like and/or a door style that is not commerically available.
Another issue to consider is weight savings which are often not nearly as great with plastics as many people assume. In fact some 1/2" plastics weigh nearly as much per sheet as 3/4" melamine. Hard to believe but it's true.
In my opinion DIY guys and gals who want BOTH weight and $$ savings should learn how to work with 1/4" Sintra or similar brand of expanded PVC. And shop carefully as the prices of these products can vary dramatically. However, 1/4" expanded PVC is not always suitable for racks as it is too flexible.
Just for reference, one of the lightest brand of expanded PVC weighs 1.34 lbs. per square foot. 1/2" baltic birch plywood weighs 1.72 lbs. per square foot and that's on the heavy side for 1/2" ply. And keep in mind that any expanded PVC product has a foamed interior which decreases the weight quite a bit. Any solid plastic is therefore going to weight quite a bit.
Other 1/2" expanded PVC can weigh 2 lbs. per square foot which is even more than 1/2" plywood.
I don't blame you at all for wanting something other than melamine cages and racks. However, just be warned that if you're not careful you might end up with a cage that is both expensive and heavy. Pick your product carefully.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)