I think the potential can go even beyond that. Not on the full scope of clients, of course, but the most active ones could end up spending more than 25 bucks a month (other will always use free stuff only). Some people spend hundreds on microtransactions in games every month (which I find unfortunate as it is clearly a predatory behaviour from publishers using people weakness against them) so we could have heavy users here as well. What is nice is that our users will be able to look at their ROI (quite different from my gaming example above): if one spends 100$/mo in sampling and observe it leads to 1000$/mo additional sales he has much incentive to increase his spending in a rational way.