The typical cost to rebuild one of these motors is expensive. These motors are very picky and want a machine shop to align bore and check every piece in every way possible. Then all the bolts in our motors are Torque To Yield, this means they are expected to stretch and this makes then not be reusable.
That being said, let me tell you the good about the 3800 motor. This is an extremely durable motor with a long lifespan. If you replace the plastic lower intake manifold gaskets with the latest GM Dealership aluminum offering (and UIM if naturally aspriated), you will likely have nothing more serious than a valve cover leak for years and years. The piston rings in our motors are a low pressure ring that barely wears to be noticeable over 200,000 miles. At the 200K type of mileage it is common to take a motor apart and see the cross hatch honing marks still in the cylinders. At your mileage I would not have any engine worries. The transmission on the other hand can typically show signs of needing a rebuild or replacement in the 150k-200K mile range if it has been treated well.
In your shoes, I would be thinking/planning for the tranmission by 200K and think of the motor as my lifelong buddy. Actually..I'm in that situation right now. At 176K miles my Regal needs a transmission and the motor is strong as ever, just put rockers on it for added lift.
00 Regal GS: All kinds of cool stuff.