Asking for least expensive options both airlines and routes from SF bay area airports to Jackson Hole,
Wyoming and nearby (larger/major) airports - renting car to drive to Ennis, Montana

It's not cheap to fly into Jackson or anywhere in Montana. For Ennis, you can try looking at Bozeman, Jackson, Missoula. Also check flying in to Idaho Falls. You will probably not have a direct flight from SF.

Also, if you are not familiar this website is very good for finding fares and routing:

kayak.com

There are others, but this is what I use to get the general idea.

For Ennis, it doesn't make sense to fly into Jackson, WY.
Bozeman MT or Pocatello ID are closer and are pretty much equidistant from Ennis.
Next I'd try Billings MT. It depends on what flights are available from your airport.
Probably the least expensive option will be Salt Lake City, but it will be a longer drive.

Jackson is usually the most expensive option all the time - and it's not a great airport option for Ennis.

Check Bozeman, Pocatello, Billings and Salt Lake City in that order and choose the best option of cost and drive mileage.

Also check here: http://matrix.itasoftware.com