how much do you guys think a good price to replace the engine on pontiac grand prix 2006 3.8

Chevy crate motor $2,100, Labor - at least $1,000 and likely more. http://www.gmpowertrainwarehouse.com/89017670.asp

Add in misc. parts and you will easily hit $4,000.

In other words more than the car is worth. Unless the car is something special replacing an engine is rarely worth the money spent. At the end of the day it will still be a $3500 car. Cut your losses now and start shopping for a new one.

Finding a used engine would be cheaper but still more than it makes sense to put into a 10 year old car. Can the engine currently in the car be repaired? What is wrong with it? Spending $1000 on a repair might be a better path to take.

It caught on fire