I have a client that wants to allow their clients to purchase say $150 worth and then they can apply this to each session they want to attend, so if one session is $7 and another is $8, they have $135 left in their account.
I have implemented a 'user points' payment mechanism where we use a read-only joomla user profile field to store a 'number of credits' which can be used to purchase events.
You would need to manage the 'credits' purchasing mechanism offline or using a shopping cart addon though.