Let's analyze past examples and try to reason behind them.

Endeavour was built on time and under budget.
Why?
Because there already were some existing parts, because there already was a good knowledge of how to assemble a Shuttle. Also because there was the loss of Challenger and its crew so there was a sense of urgency to "repair" something that had been broken.
But Endeavour isn't a spaceship for interplanetary flight; it's just a Shuttle.

Zarya was built on time and under budget.
Why?
Because the technicians were highly skilled Russian ones with a great knowledge of Mir.
Because the Russian technicians cost much much less than American technicians.
But precisely, this isn't good for US jobs and skills as it's taken away from them. And besides, Zarya is just a component of a platform meant to stay close to Earth.

The Apollo program was a formidable success.
Why?
Because it had a political motivation behind it, very extensive (not unlimited except in 1966) funding and stopped before any disastrous flight might have occured.
But every rocket was a new one, the flights lasted at most 12 days and with only 72 hours on the Moon, at most.

The ISS exists but nobody likes it.
Why?
Because it doesn't go anywhere but LEO, because experiments interfere with each other, because it had already been done by the Russians (Mir), becasue it's so expensive., etc...

The Shuttle has flown more than 100 times but costs so much.
Why?
Because it's huge, very complex, dangerous.
It's reusable but it doesn't seem at all to cost less than ELV's. It only goes to LEO and can only stay up to 3 weeks at most not higher than 600 km.
But it can do lots of different missions (however , this contributes to its cost too).

A manned mission to Mars.
The fulfillment of a dream! Heu, well wait a minute....
The first time a landing took place on another world was long ago in 1969. Mars would only be the second world visited. A trip to Mars would involve far fewer flights to space and thus save money. Yes, only a few astronauts would be needed and that's it. The Mars expedition would last 500 days but it will end one day, too. Then what? No more money for Mars, no more money for any manned space program because none will be left except the Mars one.
We have no way to know if the first manned Mars expedition will automatically lead to other ones. The nature of the goal will obvioulsy change, once the first expedition is back.
Because of the time frame, before the Apollo spacecrafts went to the Moon, 15 Saturn rockets were built; that's why there were 12 astronauts on the Moon and not just 2.
Suppose the decision is taken to go to Mars. How many rockets will be built? Far far fewer than 15! Because there will be a higher confidence in an early success and because just one mission takes so long.
Clearly, going to Mars at the expense of any other manned space program means almost certainly the end of humans in space. This, unless of course an ingenious system is discovered to divide the cost of sending humans to space, long before the Manned Mars Program replaces all others. But that's anothezr story.

Raoul