yep it does cost 100 baht more (2000thb in total) but it's very convenient since they fill out the form, take all the copies for you and the photo is taken with their webcam all we have to do is sign the copies.
I did it on a visa exempt entry but that was like 8 yrs ago, I only needed a certificate of residence from immigration office and to visit a clinic and answer some questions if I remember correctly.