Prevailing wisdom on these boards seems to be ~$3500 to get it done correctly. You can also consider getting a rebuilt motor that addresses a few other N* design issues and just swapping it in. I believe the motor is ~$3000 and then installation costs on top of that. If mine go I'm going the swap route.
By the way, TexasCadillac... your headgaskets are fine. No need to go worrying about the problem if your not even having it yet. A car from a Cadillac dealer maintained as well as that one looks to have been, is probably the least likely to give you trouble in that arena. Just keep the coolant changed frequently nd youll be fine. There are some on here that like to profess Northstar doom as common as old faithfuls eruption schedule, but while it is common, its not THAT common on a well maintained car. The problem comes from cars that were neglected.
I just put a Jasper reman. in my 99 Deville. I used one of Jasper's recommended shops near me and the whole project from A to Z was $6,500. That included the timeserts,new water pump, plugs, wires (AC Delco), etc...
Hope this helps you get an idea of the cost. :bonkers: