I think it was around 12,000 Baht for 60 hours. This is for group lessons (6 in the class). My school also does 1-2-1 lessons where the cost will obviously be higher.
I don't know what the exact requirements are. However my school was willing to put through an ED Visa application if I paid for 60 hours of lessons. I do 2 * 2 hour lessons per week. Other schools may have different policies - I am not sure.
If you enrol at a reputable language school, they will do all the Education Visa paperwork for you. You'll need to supply a few documents but the school will do all the preparation. So you wont need an agency to do it for you.