Rust

[06YarisRS]

That's nasty! It's disheartening to discover such things after buying a car. I feel for you. Probably the cheapest fix would be to grind off as much rust as you can (down to bare metal) and drap the area with thick fiberglass mat drenched in resin. The mat I am referring to is about 1/8" thick, not the thin weaved stuff. My buddies and I did this type of repair in the past and it held up as long as you maintained an oily undercoat underneath after the repair was completed. If not, water can get between your repair and the remaining metal and cause more corrosion. It might also be advisable to apply a thin mist of rustproofing compond on the inner side of your repair. Usually the carpet has a backing that would serve as a barrier to the carpet fibers. Ideally, you would weld in a new floor pan, but you could probably find someone to patch it with steel strips/plates. Regardless of which approach you take, I would intermittently apply a non-drying rusproofing compound. Good luck and please document the process and post pictures of your repairs if that's possible.

Pic of the mat I'm referring to. Lots of vendors on eBay but you could probably source locally: