For large ticket items, subscriptions, like leases, may be a good way to go. Most modules in the Market are not that expensive so a subscription model in those cases is not a good ideal. If you decide to try to go with subscriptions, it should be up to the vendor to decide to allow a module to be sold by subscription. Coding in a call home for a $20 module means that the module will no longer sell for $20 so you may end up driving up module prices.