The thing about commercial plastic cages is that, with care, they last forever practically. I have Neos that must be 7 years old and they are perfect. If I go 10 years without replacing a cage, then I say the money was well spent. Same goes with Vision and the new cages being offered by the various outfits now.
On the other hand, building your own allows for sizing to your animals' needs and available room space or dimensions. Plus, using wood, you can attach branches easier or build shelves for basking. I would imagine for an iguana you would want a tall cage with large branches. I have seen a frame of 2x4s with plastic-coated hardware cloth covering the frame for iguanas. Easy to build and easy to hang basking lamps or radiant heat panels, branches and fake plants.
I have found that cabinet makers can make you a very nice cage shell for a very good price. Much of the total cage costs are in the doors/hardware and the paint or other finish.