A rather surprising but long established trope in science fiction is the prison planet, which for purposes of this discussion means any prison in space.
There are two characteristic types of prison planets. One, probably the predominant form of this trope, is celestial Supermax, the sort of place that makes old time Alcatraz look like community service. If swimming a few hundred meters of San Francisco Bay is tough, try swimming a few hundred thousand or million km of vacuum. Only the baddest of the bad get sent to this prison planet, albeit for values of 'baddest' that probably include political dissidents and the like.
In an interesting development of the theme, Salusa Secundus in the Dune 'verse also serves as a Dirty Dozen style recruiting center, on the argument that the baddest of the bad make pretty good troops. From the mercifully little I know about California prison gangs I would not trust those guys to have my back. But in the rather baroque worlds of Dune it fits, and in historical perspective is no stranger than Ottoman recruiting, which worked pretty well for a few hundred years.
The second type of prison planet is not for the baddest of the bad, but mere troublemakers - the sort of riffraff, for example, who can't make it past the second syllable of 'barbeque,' and play football (meaning football, not that other game) by peculiar rules that may not actually exist. A civilized society would naturally wish to rid itself of such people, but be reluctant to hang the lot, especially when they include a notable proportion of attractive females.
Both forms of prison planet have historical prototypes. Devil's Island is a famous instance of the first type, the Supermax colony. The second type, riffraff colonization, was used with success in the early modern era to settle the southeastern portion of one terrestrial continent - some of my ancestors were among the colonists, though my people learned how to pronounce 'barbeque,' and correctly use it for beef steaks, not decapod crustaceans. Historical accounts indicate that the same form of penal colonization was also employed elsewhere, with some success.
There has been occasional discussion of prison planets at SFConsim-l, mainly of the Supermax type, with the consensus being that it is not really a very practical solution. The problem, in a nutshell, is that you can't just dump off the baddest of the bad and ignore them. You still need all the security measures of a terrestrial supermax prison, and you have to maintain it at the far end of a supply line. Recruiting guards is difficult when the guards will themselves be stuck on the prison planet for months at a time. And this is not really an assignment for 'trusties.'
Prison colonies for mere riffraff are a better prospect, at least in settings with earthlike planets. The key (so to speak!) is that security need not be absolute. You don't really care if the occasional petty thief slips aboard an outgoing shuttle. Most won't even try - they'll be drunk, or involved with one of those winsome female prisoners, or busy protecting their farms from being overrun by bunny rabbits.
Thus the shuttle port needs only a 'minimum security' operation. Indeed, you can plausibly staff it with trusties, who enjoy the advantage of first dibs on imports, and probably live better than they did before they were sentenced there. Operating costs are therefore much lower, and may amount to little more than the cost of transportation. This will presumably be expensive, but pretty cheap relative to maintaining a prisoner for a life sentence even under minimum security conditions.
As a further advantage, considerable historical evidence suggests that prison worlds of this type can produce pretty good soldiers - perhaps not Imperial Sardaukar, but guys you'd rather have on your side than the other side.
And, finally, who knows - the poor devils might end up rehabilitating themselves, more or less.
The image at top, of a rather meta protest, comes from a blog mainly about religion. You were warned. (The blogger's name is sheerest coincidence.)