It's Simple
A Mars Mission does not require anything like the complex construction required for the ISS which had not been attempted before and boloxed up badly with the "ideal" of international cooperation.
It seems to me the Mars mission proposals are far more like Apollo than ISS. Far fewer launches are required and no complicated orbital assembly just simple docking or direct launch. We have a far larger portion of the technology needed to get to Mars than we had when we commited to the moon. It is entirely reasonable to assume we could cover a smaller gap with less money. How effectively money will actually be spent depends of course on who is running the show and how much unneeded pork is added.
Regards,
Mark