At least that's the official line from the Church of St. Zubrin
What's the success rate for on-time, on-budget delivery of large aerospace systems? Zero? If not, it's damn near zero.
One reason why I never liked the Mars Society is my perceived view of them as a cult of personality. If Zubrin says it, it must be so. Questions about his designs and engineering assumptions are not allowed. Sorry, but I've worked in and around aerospace for too long to believe a manned Mars mission will cost anywhere what he predicts. Multiply the estimate by 2 and you'll be in the ballpark. If the US government is paying for it (who else will?), then increase the multiplier to 3-5, minimum.