cheap cheap … i think they apply 35tb straight away when coming away from the airport & it increases very very slowly, although you pay Motorway tolls for a faster journey, say 2 x 50 tb … from Suvarnabhumi to Bangkok centre approx. 200 tb 30-45 mins
the only hurdle is the airline check-in staff, just blag it by saying you are crossing the border by bus in 30 days & sound like you know what your doing