I had the same issue a couple years back, but can't recall if it was 1456 or 1457. Tried a new gas cap first, then some fuel additive, nada.
The next cheapest thing to try was the cannister shut valve @ $50. That did the trick, but was a pita to install. Drop the cannister, if that's the route you try next. The brass screws that hold the valve to the cannister are impossible to get out as the heads strip.
If you don't have to worry about passing an emission test, I'd say forget about it. I ran it with that code for almost 2 years, with no problems performance-wise or mpg's.