A Dragon + Sundancer stack would be about 13.5mt which is about what Elon says he could send to mars using a Falcon Heavy. That would be around $200m to send four to the surface of mars. Expect me to use $50m per colonist in all future calculations (but prepositioning supplies on mars, which we should do, makes it more for initial missions.) This is almost pocket change to Elon Musk or Robert Bigelow. We just have to be patient. Hey, it's only been about half a century.
We're used to seeing the capsule on the top of the stack, but it doesn't have to be. The Dragon, in order to face upward would be below the Sundancer which would be on top. Both would be mated at the docking port. Everything is integrated on the ground. This eliminates both a docking maneuver and launch abort.** Real martians don't need no stinkin' launch abort. Zubrin would approve. We could do this in as little as four years.
**Actually, you'd still have an abort option, it would just be less than 9 g's.
A more integrated approach would be a short MCT.
Error: I screwed up. I realized it before I went to bed last night and hoped someone would point it out. So fixing it was nagging the back of my mind and I had a sour stomach that sent me to the toilet several times. So I didn't get very good sleep last night.
When I read Elon's statement and realized the Dragon plus Sundancer massed what the Falcon Heavy could throw directly to mars it got me excited. Never do math when you are tired and excited. While the FH could throw that combination, the mass of crew and supplies would be an additional 8.5mt. That becomes too much. You need at least another FH to refuel in LEO and the stack would also require something like the FH upper stage to continue on to mars. Then there's another problem. When you land four in just one lander, each colonist would only have about 500 kg of supplies. Assume 100 kg is water and oxygen which is about two weeks supply. They will need a tank of water already on mars within reach which means at least one prepositioned lander close by. Water is baked out of the soil using the Mars One scheme. Water and power gives them unlimited oxygen and they can pull nitrogen out of the atmosphere.
400 kg of food would last about a year which is not enough. Until they supplement that with farming they will need at least twice that between resupply. So that's two landers prepositioned and another two or three every two years unless farming does the job. Then you should have other landers with other stuff prepositioned (like plastic for at least eight to twenty farms and other tools and equipment.) It's a lot different for the colonists that follow, but the first landing is going to require costly over supply waiting for them if we want to give the colonists a fighting chance.
Update: Bob makes the excellent observation that Elon is most likely referring to the capability of sending mass toward mars. You would still need further delta V to achieve orbit or land. Both the Sundancer and [Mars One Lander variant] Dragon have a certain amount of delta V as part of their design. Basically you just need to get the Dragon near orbit (not even all the way) for it to achieve a landing. It has a more than capable PicaX heat shield and even in the thin atmosphere of mars would shed most of it's velocity aerobraking. Most of it's fuel would be used in final touchdown with only a very small amount required to give it the initial velocity change required to hit the atmosphere even before achieving orbit. The Sundancer would have ion thrusters for several uses. These would be able to swing everything around mars for return to earth or allow orbital capture either around mars or earth.
The main point is that neither technology or funding is really holding us back. Do we have the will?