Most of the colonization sketches from MS all include a multi-purpose underground component. The rationals for not building everything underground are the difficulty of doing it, even on Earth, and our lack of knowledge of Mars underground.
All that is to say that the first few batches of Mars colonist should only expect a one way ticket. We could obviously bring them back to Earth, to die from cancer here, if they wished to, but they might as well get morphine up there and keep on martianing, I know that’s what I would want to do.
See https://science.sciencemag.org/content/340/6136/1080 and https://photojournal.jpl.nasa.gov/jpeg/PIA17601.jpg.
If it's a normal +5 bonus years of risk, it seems like something that would be acceptable for such a voyage. But you'd be a bit hard-pressed to find something worth it on Earth, it would have to be something truly spectacular.