Paul, how about a roll-over option? Say you regularly use 10 GB bandwidth per month but are stuck with a plan offering 20 GB per month. You normally would barely use half ... but you could 'bank' it for one month, giving you the opportunity to 'spike' up to ~30 GB every second or third month before getting a penalty.
OK, I pulled the numbers out of my butt and have no idea how POM feels about the idea - IF they have thought about it - but would that be a solution for you? One problem might be that you'd have to have paid for the "20 GB" in the first place, but it may be time to open up a new entry-level class and expand the definitions of the "Pro" classes - maybe more storage and/or bandwidth as well as additional services, for a small additional cost.
Dave M