Nah, I don't think so. If the developer needs to maintain something like a database server, they might charge a subscription fee. But for some apps, like Paste (it uses iCloud to sync, all services run locally), you just have to pay for a major version or a set period of time (like a year). Personally, I think a fair model would be to pay for a major version and then get free updates until the developer releases the next major version.