First, let me thank you for saying up front what it is you're looking for from a cage. Saves a lot of time vs. asking what you're looking for exactly.
I don't know enough about tegus to know how much damage they can do to a cage. If you need FRP board or polyester resin to line the cage this could get expensive. My suspicion is that they need a durable protective coat, but I'm not 100% sure. If I were to get a tegu I would build a cage with an epoxy coating, no doubt. Just to be on the safe side.
Otherwise, the biggest issue with a cage this size is the amount of material it will use. Almost all material that might be used for a cage this size comes in 4' x 8' sheets.
So...
The back panel at 6'x2' and two side panels, each at 3' x 2' could be cut from a single sheet of material. Scrap piece would be 2' x 4'.
The bottom panel at 6' x 3' would have to be cut from a single sheet. Scrap pieces would be one at 1' x 6' and another at 2' x 4'.
The top panel at 6' x 3' would have to be cut from a single sheet. Scrap pieces would be one at 1' x 6' and another at 2' x 4'. The 1' x 6' piece could be used to make a face frame for the cage.
So that's three full sheets of material. If melamine is sufficient for a tegu (ask this on the tegu forum) then you're looking at maybe $120 for the carcass of the cage. You would also have some useful scrap, which may or may not be a good thing.
I have not priced glass in a long time and it tends to vary anyways. You'll need to call around and ask.
What type of door do you prefer? Sliding bypass dooors, hinged doors? You'll want to avoid plexiglass or other types of acrylic due to how much tegus scratch.
I'm also wondering if building a cage from hollow core door blanks might not work. You can often find these scratch and dent for about $10 each. Otherwise, I have found decent HC doors for $22 each.
You could use two doors at 80" x 32" for the top and bottom panels.
A third door at 80" x 24" for the back panel.
Just about any sort of scrap wood, melamine shelving, or whatever for the two side panels. Or even another door cut in half.
You would also need something for the face frame.
This would be cheaper and lighter than melamine but then you'd absolutely have to seal them with something. If melamine is durable enough then it's your best option since weight is not a concern.
But if you want/need to use an epoxy coating anyways then HC doors might be something to look into. You might be able to build a cage carcass for well under $100, especially if you shop around for scratch and dent door blanks.
The rest depends on what you want for doors, etc. This is the unknown factor going into any project. Little things like bottles of glue, paying shipping on glass track, or even paint brushes add up quickly.
Also, what tools do you have access to?