As low as maybe $800 (gears/ labor/ fluid/ etc), and I've seen gear installs at $1200+. Depends a lot on your location, and the shop itself.
Gear installs are one of those areas, that is worth paying more to a shop with a lot of experience and a great reputation....than to save a few bucks. A gear install is not complicated, but it is very precise, and should be performed by someone that has done many and is reputable. I would also ask a shop how many Jeep JK's they have done.....(piece of mind knowing that your JK is not a guinea pig- lol).
Whether or not it is worth doing is up to you. If you think your JK feels as if it has lost a lot of performance since upgrading tires (on-road/ off-road/ acceleration/ mpg's/ etc), than you might want to look into it, but if you think it feels okay- then don't worry about it.
If you don't have 33's yet- you might want to do them first, and see how it feels with the stock gearing. Some will feel okay with that set-up....others will not.
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