$640 to Jackson is not at all unusual during high season. If you can get any fare $600 or less, you are doing well. As long as oil prices continue to be high, you should not expect to see any significant reductions in airfares. While I can't predict what will happen, I do believe that this year in particularly you are unwise to try to wait it out. Once you find a flight you can afford, book it.

Two words for you "peak season".

Sign up with Kayak for email alerts if the flights drop in price but don't expect any late bargains for flights this summer.

Do remember to factor in the cost of gas and travel time from other airports!

Hoping for a REDUCTION in rates? Why?

Chances are that you will be paying more per day for a rental car from the Jackson airport than at a larger airport.
Airline fares are very dependent on the oil market which has been going up steadily.
Don't be surprised when you pay over $5/gallon for gas in Jackson and maybe more than $6 inside the National Park.

It is a pretty long drive from SLC or Denver to the Parks. Better just suck it up for Jackson, and I wouldn't wait. You could also do your car rental now--it will only get more expensive as time goes by.

You need to price out the total cost of flying into each airport. Airfare is only part of it. Factor in the rental car price, plus the cost of gas to drive from each airport to your destination(s). And don't forget baggage fees - Southwest gives you 2 freed checked bags, other airlines could add $50 or more roundtrip (Southwest flies to Denver or SLC but not Jackson). Also consider what your time is worth - if the trip is short you don't want to waste a day driving to/from an airport. There's more to it than just looking at the airfare alone.

Some airlines and rental cars will refund teh money if faares go down...not that I expect them too go down this year, but I have had success with Delta and with all the major rental cars.

There is a rental car site that tracks prices and will notify you if the price of your rental car goes down (can't think pf teh name of it right this minute..maybe some other Foderites know) ....farecompareis good about notifying you re: airfares in certain major air hub destinations

I would also try Billings. You could always drive the Beartooth that way. Maybe also check a couple of other spots--Bozeman, Cody, West Yellowstone . The airlines is crazy, so you never know until you check everywhere. Also, try changing from where you are flying out from, if that is an option.

Where are you flying from?