The exact cost of a Mars mission would depend on the nature of that mission, though most of them likely have a minimum price tag of hundreds of millions of dollars. An unmanned capsule such as SpaceX's Red Dragon could theoretically make the trip to Mars for about $750 million, while a larger manned space mission could cost upwards of $100 billion.
Mars missions don't necessarily have to be extremely expensive. In September 2014, India's space program made headlines for successfully launching a Mars orbiter that cost a total of about $75 million to produce and launch. The fact that the orbiter is not slated to land on Mars helped to keep the cost down.
It's likely that the cost of a manned Mars mission would not be paid by one company or country alone. NASA, for example, would likely collaborate with several other space programs, public and private, in order to complete the journey.