According UMass prof Ray La Raja, Congressional challengers who lost but held incumbents under 60% of the vote were outspent by an average of $894,000 in 2004 according to Campaign Finance Institute data. This was a significant increase from 2000 and 2002. Thanks campaign finance reform!

(For more on the problems with campaign finance reform, see here, here, here, and here. Of course, other factors could account for part or all of the gap, but campaign finance reform appears to play a major role.)