1991 Corolla automatic transmission

The dilemma: Should I spend this much money on this car.

Story: The trans went out this summer. My son tried to save me money, by doing it himself. This was his first automatic transmission, that he had replaced. He did not know that he needed to add fluid to the differential, separately. I drove the car for about 5000 miles with no problem. Now the error has been discovered and I need a new transmission. Car seems to be in sound mechanical shape, but has a lot of rust. I am trying to decide if I should put more money into this car. My son is no longer able to work on the car and it looks like a total of around $800 to have another transmission put in, with only a 90 day warranty. I would greatly appreciate any input. Thank you

More info: car has around 126,xxx miles. Rust around bottom edges, two large holes in the trunk. Mechanic says that the rust has not affected the integrity of the car. Engine. steering and suspension seem fine. New struts a year ago. Keep the input coming . Thank you.