Presuming you don't necessarily mean that, but this is
/. , so have to examine.
Let's assume that the prisoners in question are young (so as to be physically capable of the trip), so we'll start at age 25. Let's also assume that their life expectancy in prison is 50 years. So, the math at that point is fairly simple, so long as we don't calculate for the additional expense of medical care that geriatric prisoners incur in their waning years.
Using the report at
http://www.dc.state.fl.us/pub/statsbrief/cost.html/ as our guide, let's assume that your average prisoner costs $55/day to house and feed.. So, discounting any other costs and inflation, the cost of keeping an average prisoner in jail for 50 years is around $1,000,000 (based on $55/day * 365 * 50).
Not sure, by that standard, that it would be cheaper to shoot them into space on a one-way trip to Mars, given the cost of fuel and the various other things to keep them alive. I mean, yeah, you get free labor and all, but unless the plan is to send them up as mere lab rats or unskilled labor, you'd presumably have to teach them to do stuff that they may not know, unless you pick an exceptional prisoner (someone w/ an
MD or something like that..
Still, probably a good cost:benefit ratio, all told.
