If done correctly, the existing caulk is removed, and new dicor self leveling caulk is applied. Most manufacturers say to do this on a yearly basis. You can also apply a new layer over an existing layer, but it is generally suggested to only do it once like that.
It is a bit tedious to do, but can be done if you are decently handy and feel up to the task. Materials wise, you can expect anywhere from 8-14 tubes of self leveling dicor. It will vary according to how heavy you lay it on, and how many items you have stuck through your roof (vents, etc).
I have done a couple. I personally went with eternabond tape over all my seams, so I never have to worry about the sealing on my roof again.
Materials for eternabond will be less that what you will spend at the dealer. Plan on a couple of good weekends to apply it correctly, but it is so worth it in the end.