xrefer adds paying services: "100 books - 21 publishers - 1,250,000 entries - over 5 million links - fully searchable - fully browsable - fully cross-referenced. xreferplus is the new digital reference library from xrefer, makers of the web's reference engine."

Pricing is based on the size of the subscribing institution and the number of patrons the library serves. Link found on the Virtual Acquisition Shelf & News Desk.