Zotero just announced a new free storage feature for attachments. I blogged about it over on the library's site, so I won't repeat it here.
The first 100 megs of storage is free. I've got a fair-sized library, and I'm only using half that. I probably won't need to upgrade to paid storage until well into next year, but you can get a gigabyte for twenty bucks a year. That strikes me as quite reasonable. I'd pay twenty dollars for another gig in my Dropbox in a heartbeat.