Eh... incorrect about the comparison to Earth. The issue isn't the very thin atmosphere but the lack of a strong magnetic field. Despite being twice as far from the sun, in orbit around Mars you receive 2-3 times the radiation exposure as you do on the ISS. That's not going to be dissipated much on the surface.
A much larger issue is any solar flares that passed by Mars. I wouldn't want to be there for that without some serious shielding. The again, I'm not sure why I'm assuming you have the technology and money to get to Mars but not the foresight to build adequate shielding. Probably not as big a problem as I first thought, but if something went wrong... somewhat scary.
Having checked my numbers, you're right, it is quite a bit more than that, but still nothing to worry about. A quick wikipedia check says that average radiation doses on the ISS are around 150 mSV. Estimated doses for an approximate 900 day Mars mission (180 days travel each way, 550 day surface occupation) is about 520 mSV. So about 40% more radiation that you'd see on an ISS stay of about the same length, but that's still not very much exposure: it's a 1% increased cancer risk over 30 years.
That does somewhat disregard solar flares, but solar flares are fairly weak all in all; particles on the order of millions of volts. You could shield against those easily just be being in the middle of the spacecraft, surrounded by the vast amount of supplies you need to drag with you. 30 mSV exposure probably.
Isn't this supposed to be a one-way ticket? I doubt the radiation is enough to actually kill anyone outright, and who cares about cancer if you're not going back anyway?
It could be one way, but it really doesn't have to be. In the case of the first few missions (that you'll need to build up any sort of base), it will almost certainly be return trip.