White papers traditionally can be almost any length. I've written some that are 3 pages long, others that are 15-20 pages. Each with variation in technical information, ranging from "Here's what's important to know in this knowledge domain" to documentation of a proprietary programming language.
While some of these are written by analyst firms, quite a few are not. They're work-for-hire, often by techies or journalists who know the area in great depth, and who can ask the in-house experts the right questions and turn those answers into something genuinely useful. (Well, when they're good. Mine are good. :-) )
>to documentation of a proprietary programming language.
We're probably just quibbling over terminology. The people I know use whitepapers to mean more business-oriented, less vendor-specific thought leadership pieces. And use other terms for technical deep dives--which can indeed be longer.
However, when you get into the $10K-20K price for whitepapers, you're usually not just hiring a writer. You're typically paying in part for a research firm's reputation and, possibly data.
I'm not sure how much we pay for straight writing under our logo. But it would be significantly less unless it came as part of a package with advertising etc. as deals through online news channels often do.