What type of visa
are they getting you for one year? After you have the visa
you can apply for a multiple re-entry permit for something like 5k at immigration without their help. Are you sure they aren't just offering to do that portion for you?
I have a friend who used Siam Legal for his wife and children who go to school here. They charged him about 30k and the wife still had to fill out all the paperwork and go to immigration with their 'helper'. He wasn't thrilled but didn't know how to do it himself.
I got my triple entry converted to a Non B for learning Thai (good for 90 days) and a 1 year (no need to leave every 90 days) visa
extension for a total of 15 months and I didn't need to leave Thailand to get it converted or do visa
runs, just 90 day reporting. This is done like a University student... Not a risk everyone wants and costs 30-35k (also got a proof of residency paper for my 5 year Thai drivers license) but it sure beats the visa
runs which I was getting tired of