This just requires a couple of additional inputs. One to specify a number, and another to specify a unit of time. For example, if I want to sell a subscription based product via the Resource Manager, and it is $5 per month, I would select 5 USD as I normally do, but there would be an additional input to add that it was a per month (or indeed day, week, year). This would serve to give a more accurate representation of what the user is purchasing. Taking it to the next level would involve stipulating that the original purchase price is X, with an additional cost of Y per year/month, etc.