According to this PDF report from the Chesapeak Digital Preservation Group, 44% of links go rotten. (The Chesapeak group is comprised of four member libraries, the Georgetown Law and Harvard Law School Libraries, and the State Law Libraries of Maryland and Virginia.)
N.B.: There is a high value in preserving non-trivial content in ways that allow reliable access for indefinite time at a permanent online address. (In theory search engines could provide this as an additional (and optionally paid) service to final users who request it, as this is part of what they already do by default.