For what it's worth, The Foundry have a system for their subscription wherein you can allow it to lapse but if you wish to re-new it later on (for example after a significant new release) you have to back-pay for the entire time since you first let it lapse. As such, it's no cheaper to do it that way but it's also no more expensive - meaning that if you never find another feature that you think is worth investing in, you never have to pay again. If you do, then you do. IMO this is a fair system to pay for the development.