Good luck finding the raw materials for less than the cost of a premade enclosure. PVC sheets are ridiculously expensive and quality plywood is not cheap. If you have access to someone that can build a wooden cage or you have the equipment and resources to get materials it can be cheaper but say if you live in an apartment and don't even own any power tools then you are not going to save any money trying to build your own.
This comes up every month on here and the answer is always the same... unless you have access to what is needed to build your own, it is never a cost savings to do so. Maybe 10% of the people on this forum have the equipment and ability to build their own cheaper than buying from say AP. There may be a few people that try and convince you it is doable forgetting they have access to tools and contacts you do not.