I dont know if this works cause I have only ever used it when snorkeling.... but....

We used to rub a bit of toothpaste into our snorkelling mask and then give it a quick rinse out to get rid of most of it and then your mask would be fog free for an hour or so...

Dont know how minty you like your camera but this may be a cheap alternative to try....

Like I said, I have not tried this in a camera, but it sure worked on a face mask while snorkelling.

Cheers, Luke.