I have both made and bought cages. So far my coolest cage that i own was made by me, it's 6'7 X 3' X 3' housing an adult veiled cham. He has lived in there since he was about 3 months old. Not knowing then what i know now, i would have caged him in something much smaller than that, but you live you learn and he is now big enough to use this cage to his advantage. The cage costed me around $75 to make. I'm going to post pics of it in the near future.
Rest assured though, that if you're going to buy a cage it will most likely be perfect. There's always that margin of human error when making something on your own. My vote is give making a cage a shot. It was a very fun and rewarding experience for me.