Figure about $125-150 to have an auto upholstry shop do the seat, I am not sure if you can find the pr-cut foam at the dealer or online.

If you have seat covers, you might look for a passenger bucket seat from '84-89 4-Runner or '84-85 SR5 extra cab and swap the seat back and bottom, most passenger seats are almost new. This would be the cheapest way to go since you are buying one seat and it would be a direct bolt in.

Otherwise, pick up a set of used buckets at the wrecking yard, look for Toyota seats for better fitment. Measure the height of your seat(floor to top of cushion), and try to stay within that height, alot of newer buckets are taller which may reduce headroom.