How much would it cost to repair/replace a crack on the Flex pipe?

A decent exhaust shop should be able to cut out and weld in a replacement flex joint for a reasonable price. Or just buy the replacement front pipe with it all in one and replace it that way. There were two engines available so two different styles of pipe as well. If you have the 3.5 v6 the replacement would be easier than the 3.7.
For the 3.5 you can use a Walker - 55693 front Y pipe like the first image If it's the 3.7 it uses the Walker 54982 with 2 pipes and flex joints like the second image.
I would look underneath or on the sticker under the hood to verify which engine you have. Then talk to a couple exhaust shops and see what prices you get. Price wise it's hard to say but I would expect an hour of labor and the price of the part. In my area that would be about $350-400 depending on which engine it is and which pipe was used. To cut out and weld in just the replacement flex would be cheaper, likely under $200.