How much is paid is not the only issue here. When and how it is paid will make a big difference too. If it is paid into an interest-bearing account back on Earth at regular intervals, then obviously they will accrue a fair amount of interest on top of their base salary. If they get it all up front and are able to invest it, or have someone invest it on their behalf, they are going to be very well off when they get back. If they don’t get paid the agreed-upon salary until the mission is over though, they are getting screwed big time since the inflation of 20 years will diminish the value of that money considerably.