The above chart comes courtesy of the Committee for a Responsible Federal Budget, which collates various banks and other analysts' predictions of how the economy will be affected by a shutdown. While there are some outliers (Mark Zandi at Moody's and Ethan Harris at Merrill Lynch seem to be the resident pessimists) the consensus is that a one week shutdown will cut fourth quarter growth by 0.1 or 0.2 percentage points.
That's not a ton, but it's a blow to an already weak economy that could use all the growth it can get. If you think the shutdown will result in substantial improvements to policy, then it's probably still worth it. But the odds of either side giving up anything don't look too high at the moment.