I used grills for a long time and didn't experience fog with my stock lens for the most part. However, it doesn't get very humid up here.
When you was the inside of the lens, use cold water, and try to let it air dry, or pat-dry it with a microfiber. If I were you, I'd buy some fogdoc here http://fogdoc.com/#
It's alot cheaper than a new lens, and works great.
When you do decide to get a new lens, go with a dual pane. When you clean a dual pane lens, don't use water.