The 340-600 is supposedly 8-10% cheaper on a cost/ASK basis, the other variants I presume would be more expensive due to the lower number of seats. However there are several factors that can fudge these figures, like actual seat config, stage length, actual airport charges, etc. Eg. if a 744 is fully owned and already fully depreciated, then it may make sense to absorb the higher direct operating cost in exchange for zero ownership cost rather than replacing it with something that may be marginally more economical to operate, but will require cash out to own. Aircraft replacement decisions are seldom as easy and straight forward as what is presented in the glossy A or B sales brochures.