The operation of bringing the first four people to Mars would cost 6 billion US$, that is the cost for all the hardware. For every next Mars missions or expeditions including the hardware and operations cost, Mars one will cost 4 billion US$
But according to the International Business Times, in 2016 Musk told a crowd( a big group of people)at the International Astronautical Congress that early SpaceX estimates place the cost-per-passenger at around $10 billion. Hopefully, that includes free cocktails and HBO. Musk is an optimist, though, and he seems to also think that eventually.