Most schools provide free accommodation to volunteers. Many offer free meals, as well. So the main thing you’re paying for is an introduction to a school. To put things in perspective, most teachers with a university degree and a degree from a teachers’ college earn somewhere around 15,000 THB (Thai baht) a month. That’s about 500 U. S. dollars a month or about 300 British pounds. If transportation is included, know that modern, air conditioned buses will take you on a 6 hour trip for about 300 THB. All night trip from Bangkok north to Chiang Mai or south to Phuket might cost somewhere around 800 THB. Air conditioned vans travel from most communities to the largest nearby town or city at extremely reasonable rates. 

So if an agency wants 20,000 THB or more per month for the privilege of volunteering at a Thai school, they are likely trying to cover their operating and marketing expenses in Europe or North America. But the cost of services, on the ground in Thailand should be very minimal.