@Salvador ******************
I think it was about 1500 baht. They took the picture, took copies of all the docs they needed, filled out the forms and went to immigration to get the letter. To me it was worth the money but it can be done cheaper if you are willing to go yourself.
I used an agent for peace of mind as I wanted it done quickly. It is well worth having a Thai Bank account if your planning on spending any time here.