Why does my Jeep's engine rev up and down when I start it cold?

I have 2006 Jeep Grand Cherokee Limited. When I start it in the morning the engine doesn't idle like it used to- it revs up and down for about 45 seconds until it stabilizes. The car has 65,000 miles on it. Is this a problem? If so, what is the fix and what will it approximately cost?|||It could be your transmission fluid or it could be a worn gasket. It could range anywhere from 50-500$