Technically, DropBox is using the Freemium approach. You have a certain level where it's free, but if you start using more than the average user, you are asked to pitch in a bit.
Dropbox does this with storage space, but there are a lot of ways you can actually track this (Maybe bandwidth is a larger cost for you, charge based on that).
This is one thing that boggles me about so many people doing startups. They have a good idea, and want to start a business around it. But they don't like the idea of charging for it.. At this point, your only hope of ever making money from your idea is if a big company buys your technology to integrate into their systems. But this will only happen if your technology is novel enough to warrant patent-ability. Yet despite all this, investors still flock to these "companies" (I use quotes because a company is formed to make profit from a product. If you're not charging for it, and it's not monetized, it's not a company, or a product really).
So what you need to ask yourself at this point: Do you want to develop a cool technology / idea, or do you want a company. This will determine how to monetize your idea. But honestly, without having a good knowledge of how your technology works, and what your business is doing, we can't really answer your question about how to monetize it. Why not just copy DropBox's model? It's obviously working, why fix it?