TROs have been mentioned in the past as one of the higher selling products (don't recall if it was stated as the highest). Specifics on highest selling books haven't ever been revealed, and likely never will be. Also, there are several things that don't change in terms of development for PDF vs print: writing, artwork (that's always a significant cost), and time/pay for the layout folks. Compared to those three, it's not hard to imagine that printing and distribution costs are not as significant. And while it is great to think that having forum fans help for free would solve part of the writing costs, there's legal issues wrapped in that scenario. Writers and artists still need to be paid, and layout folks still have to be employed/paid (no idea if they're full-time employees or freelance), doesn't matter if it kills some trees or just some hard drive space. On the resource side, all the same work needs to go into a PDF product. It still needs to be proofread, edited, and fact-checked. In the case of Handbooks, the fact-checking has been noted as particularly intensive. While some of those may not cost money directly, they will certainly eat up resources and time in the product pipeline.
All that being said, I'd prefer to see a Clan Handbook myself, and I don't even really consider myself much of a Clan fan. But I can understand why it is not currently happening for an under-performing product series. I still wish the Starterbooks were being produced though, so I get the pain here. :)