What site directed you to HubPages? Did another site offer you payment if you commented on hubs or filled out surveys?
Otherwise, there is no fees or subscriptions for HubPages. You don't make much for your time either, unless you make this your full time job. Even then, it appears that it can take months to develop enough of a following to make any real money from Google Adsense or Amazon Affiliates. If a lot of work qualifies as a 'catch,' then that's the only catch to HubPages.