They offer a large number of even smaller niche services; you're sort of expected to make your own bundle of subscriptions that apply to you. But as you may guess by their name, their flagship publication is the quarterly one. Yeah, their big moneymaker is published just four times a year.
You know how much CQ charges for their services? Hint: this is their subscription page: http://corporate.cqrollcall.com/wmspage.cfm?parm1=79 Yes, that's like 30 different phone numbers you're looking at. No, they don't let you subscribe online.
When that happens, you're either dealing with someone living in the stone age or a hyper-niche publication that everybody who's anybody in the world of Federal politics subscribes to, despite its absurd pricetag (e.g., four figures for a one-year sub to just Quarterly). That subscription isn't a status symbol, it's a necessity. Their livelihood depends on the information Congressional Quarterly provides.
It doesn't matter how small your market is. If your product is only of value to a slight fraction of the population, then make it so good they can't live without it. And charge out the ass for it.