It can be a one-time payment, if you know how economy works. You can get DACs (Basically Digital Currency with to value but increased playtime, but can only be purchases or traded) to extend your gameplay time, and you can never lose a DAC by dieing or anything. Once you have 1, it can only given/get by trading, ransom, ect.
But no, SP is a *no offence* horrid idea. It would greatly disconnect the community, allow piracy, and a whole other host of problems.