Roger Pielke Jr. has an interesting chart on the cost of the recent drought, compared to past years. This year's hardly compares at all (at least, on a national scale).
You could also scale the numbers in terms of inflation, as measured by the annual CPI. I've give those in the table below.
In USA Today an expert says this year's cost will likely surpass last year's, but it would have to be more than 4 times larger to equal the cost of the drought in 1980.