Here is a very useful Taxi Fare estimator site.
http://www.taxifarefinder.com/main.php?city=LV&from=McCarran+International+Airport+(LAS)

It not only shows you what the estimated fare should be, but it shows you a map of the route as well as an itemized list of the charges including an estimated tip.  It allows you to see 3 suggested routes and it indicates which one is cheapest, fastest and/or shortest.

The link is specifically for Las Vegas, but the site covers many other cities as well.