Read "The Case for Mars"
The answer is, send a crew of four, for a long trip (600 days on mars), for a 3 year trip time total. Send an earth return vehicle first. This ERV will have a feedstock of hyrdrogen, and use the CO2 that exists in abundance to produce fuel for the ERV to fly home. Once the ERV is fully fueled, launch two rockets to mars. The first rocket contains the human expedition. The second rocket contains another ERV that can be used as a backup for the original ERV if something goes wrong with it. All three rockets will include a pressurized rover with a 1000km range, so even with really BAD flying/landing, the crew could get to an ERV. And even if they couldn't, their habitat module can support them long enough for an emergency ERV to be launched.
Edit: Oh yeah, all this technology exists today or can be adapted from existing technology. No more than $50 billion for the mission, and that's over twice what Robert Zurbin projected in his book.