the prelude was based on the accord, and the tsx IS an accord. they most likely use different platforms based on the size requirements of the car, not cost. and when did the accord platform become expensive? the accord platform is the basis for so many honda automobiles right now (accord, tsx, odyssey, TL, MDX, pilot...). using it for one more car isn't going to make the cost skyrocket - in fact it'll prolly make it cheaper.