Despite what other comments say, no one is paying for Westlaw or LexisNexis per hour, and attorneys aren't forced to search as quick as possible. Most firms will have things like all the cases and statutes in their jurisdictions in their subscription, which allows them to search/view them as much as they want, with no additional cost. Occasionally you'll want to view something that isn't part of your subscription, in which case you go "out of plan" and pay to access that content. From there you can either pay per piece of content accessed, or by the hour, but everyone who is cost conscious just pays per piece of content. A lot of junior attorneys don't realize that they can change their settings to pay per piece of content accessed rather than per hour, but that changes once they get their first huge bill for going out of their subscription.
You're not just paying for the cases, you're also getting access to secondary sources like treatises and law review journals. The real value in having Westlaw or Lexis is that they have attorneys summarize the cases, classify them as touching upon different points of law so you can easily find more cases that deal with the same issue, and also let you know if the case has been distinguished or overturned by a newer case.
Source: I'm an attorney who uses Westlaw