Three things that I do:
1) Make it clear to my friends that I prefer less expensive activities. We can hike, we can picnic, we can have a game night, etc, etc, etc, but I'm not particularly interested in going out to eat. Because they know this, they're less inclined to invite me to dinners and things that they know I don't particularly enjoy.
2) "Pre-game" when I am going to a restaurant. I try to eat before going out to eat so that I can order something less expensive, like soup. It works.
3) Use the "Previous engagement" excuse. I have absolutely no problem turning down expensive plans by telling others that I have a "previous engagement." I mean, yes, the "Previous engagement" might just be plans to walk my dog, but I can still claim that it's a plan!