If you're preaching out, guest speaking at someone else's church, it doesn't cost anything to be polite. You know the pastor is going to pick up the check - when I was an evangelist I didn't pay for one meal for five years - but it doesn't hurt to offer.
I always kept a ten dollar bill in my pocket. I'd pull it out and offer to pay my part of the check. One pastor said, "Oh put that up! You've handed that same ten bucks to every pastor in the section. If I actually took it you'd have a heart attack."
Another thing that doesn't hurt is to do the courtesy reach. While the pastor is reaching for the bill, you reach for your wallet... "Oh, you're going to... Thank you." I don't even have my wallet back there, I just reach.