I think number of pages is a good metric for pricing plans. Maybe the numbers could use some tweaking (e.g. something between 200 and 50k), but this seems like a good differentiator among people and how much money they have to spend.

It also seems like a good way to divide people by their scale. You want a metric that means something to a customer, and one that they will bump into eventually.

What you could do to add value to the upper plans, is also offer hourly scans/on-demand scans/scans invokable by an API for the big customers.