I agree, it depends also on what materials you have at home vs. having to buy. For example, building a wooden cage is great if you already have nails, wood glue, wood sealant, etc. but if you need to buy all that stuff, you've very quickly gone over-budget. Just a good type of sealant seems to run for $20 or more for a little jug of it.
I've built my own cages and think buying the screen ones is cheaper. I would, however, prefer to spend the extra money and pay a professional to make beautiful cages for me but where my budget stands, buying screen cages is better.