If the 20k includes everything but the chassis, I'd say that's a pretty good deal. If they're using a brand new LS3 and a rebuilt T56, that's more than 10k dollars right there. Another 6-7 in other parts and 3-4 in labor. Sounds about right.
People selling well done swapped cars for ~20k dollars are generally losing money, or at best breaking even on parts and losing their labor.
I charge 1000 dollars just to do wiring, and probably 300-400 in labor. Now, that's about 33% of a full swap's effort, but still, 3000-3500 for labor on a swap is pretty cheap, that's only 60 hours of labor at 60/hour. That's two guys working on a car for less than week.