CELTA trainees vary from those who want to try a new career and have no previous teaching or training experience, to qualified and experienced teachers (including some English graduate teachers) and trainers who want to specialise in EFL/ESL/EAL, or who are applying for posts of special responsibility in a state school and need the TEFL certification. Yes the course is intensive if you do it over 4-5 weeks and the people with prior experience may have a slight edge over those who were stepping into a classroom for the first time. But you have some useful experience too. There are ways you can prepare. Get hold of the reading list and borrow or buy the books. There are also two CELTA books specifically written for both trainees and trainers. I would say that all of the books would be useful reading or reference for any TEFL course.

The difference between $2000 and £1100 is not that great, but you will have to be assured that the TEFL qualification you choose has some merit. With the exception of Trinity and CELTA, the field is not well-regulated and there are some dubious providers. You should certainly try to get more claasroom experience as, from what you say, your 18 months has not been directly classroom-based but correcting students written work. This should help you with the grammar components of the CELTA course at the very least.