Repair estimate

It all depends on where it is located. If yours is behind the instrument cluster next to the speedometer cable fitting, you can expect a bill for at least 1 hour labour just to take your dash apart and put it back together.
What makes you think its the VSS. By the sound of your post something more catastrophic is happening.
Seek a reputable tranny shop that will give you a no obligation FREE estimate.
Chances are your tranny is cooked and needs a desparate rebuild. When was the last time the fluid and filter was changed?
If money is tight consider buying a used tranny from a recycler with a min 60 to 90 day warranty.
Change the filter and fluid as soon as you get it in.
The fluid is so vital in these chev trannys. Like engine oil after about 75000 miles it no longer lubricates effectively.