fair adj. 8 [only before noun] used for emphasizing that an amount, size, number, etc. is large: By this time she had saved up a fair amount of money. (Macmillan English Dictionary)However, other dictionaries define "fair" as meaning "moderate," and it's easy to find examples of that meaning, too:
Some of that exposure probably came about because Target has spent a fair amount of money - executives won't say how much - exposing the brand to the New York audience through subway placards, "wild" postings at construction sites, water tower wraps and a gigantic billboard on 42nd Street. Generally, that kind of marketing campaign is pretty expensive, said Rao, of the Carlson School. (Minneapolis Star Tribune, December 22, 1999)
fair adj. 11 of moderately good size [a fair fortune] (Webster's New World College Dictionary)At least one dictionary, the Shorter Oxford (but not the full OED), gives both meanings:
fair adj. 3. moderately good, large, or satisfactory; not undesirable, but not excellent: a fair income, appearance, reputation (The Macquarie Dictionary)
How much confidence do you have in the ability of the U.S. government to prevent further terrorist attacks against Americans in this country: a great deal, a good amount, only a fair amount or none at all? (Washington Post-ABC Poll, September 11, 2001)
fair adjective. 5 Of an amount, fortune, etc.: considerable, handsome. ... 17 (Of degree or quality) moderate, adequate, reasonable; (of an amount etc.) not excessive but sufficientDictionaries—especially those for learners—should note this ambiguity of "fair" more clearly.
(February 3, 2003)