I use only Chob Panma. I am not sure of how you distinguish private vs. guided since a private tour you have a guide and they guide you, hence it is a guided tour.

They charge for Bangkok only 2,000 Baht for the day, the guides are bilingual and certified (my companion is Thai so the bilingual part is important). All day to Ayutthaya was 6,000 or so and that was a van, driver, guide and 9 to after 5 p.m.

You pay for your food and admissions. Which in Bangkok are not much. I always pay for the guide's as well and tip at least 500 Baht on the 2,000. The guide rarely has to pay for admissions anyway, as a guide and/or as a Thai.

They have packaged tours, e.g. planned around an agenda which they are quite experienced with but we go off the plan regularly and lately the tours are pretty much made up by me and they do the guiding. I have used then four times in 2 years.