To do it right you'll need to consider several things.
First, you'll have to source the crank. You can used a JY 4.2L, you can get a reman crank from any parts store, or you can buy new. I have new cranks available for $200.
The rotating assembly starts at $1150 for rods, pistons, rings and pins. You'll need other stuff as well (special bearings, custom balancing, etc.). I build many of these 4.2 stroker kits and I keep most everything on the shelf ready to go.
Some of the stuff is custom (pistons are made to order based on desired compression ratio, etc.), and the crank needs to be balanced to your exact final piston selection. I offer all of these services.
Most of the time you'll find that a properly built shortblock is going to cost about $2000 (just a round number) for parts and another $1000 for labor. This varies and it's not all apples to apples (for example does your local shop have a torque plate for the 3.8L engine?) and so on. I encourage you to look around so you can be an informed shopper.