You realise there are already people living in appalling conditions on Earth - extreme poverty, overcrowding, malnutrition, etc. Sure, they may not be your first choice for colonisers, but consider that a lot of historical colonisation was done by people in similar conditions. If you offered families currently at risk of selling their children in to child marriages / child labour an alternative of a few thousand dollars and their child being sent to Mars I imagine you could get a huge amount of takers very quickly. Look at the risks people are willing to take to flee places like Syria at the moment for the chance to try and build a better life for themselves. Additionally, space travel has such a huge cost because of the tiny margin of failure that is considered acceptable by the population in the developed countries capable of launching rockets. If you dropped the success target to 95% or 90%, you could probably strip out a huge amount of expensive redundancy and engineering - and offer some extra danger pay, or adjust the media perception of how bad a failure is, and you would probably still get a huge number of signups. Hell, if you offered me a free ticket to visit the space station with no danger pay and a 90% success rate I'd probably take it right now.