If the design of the system is good, the overhead of stocking is going to be cheap. I mean, communications to the box says "put these discs in these locations", the guy opens the box pulls out that "tray" or whatever (many possibilities here) and replaces it with another that's also been computer and roboticly stocked, you're done.
Maintenance is a bigger unknown to me, but seeing as how they're in their 3rd? generation, they seem to have that under control. Capex could be the big one, those machines and their DVDs aren't cheap ... but they've been keeping this game going for quite a few years. A serious analysis of their financials would be required to make a guess of that, assuming their public owner breaks them out sufficiently.
Ah, and that company does the Coinstar coin cashing kiosks, so that might improve their servicing situation.
I do wonder at what point the overhead of stocking the machines exceeds the income earned by having one conveniently located to pretty much anyone. Can they afford to keep them stocked with all the latest videos when they have one at every gas station and Walmart in a state?
On a related note we recently tried out the new redbox streaming app on the PS3. It was not a... smooth... experience. Errors galore, images failing to load, and actually getting the video you bought to play is pretty much a roll of the dice. I think their servers were woefully unprepared for the load they were subjected to.