In 2006 my Onan Microquiet 3600LP cost $4,100 for the total cost. Generator and installation. I don't have a transfer switch, the shore power cord is simply moved to the generator powered receptacle.
I hope to never have an LPG generator again. Under continuous use, expect to go through 60 pounds of LPG in 24 hours.
The next TH I am considering provides a 30 gallon gasoline tank for the generator. Gasoline is much easier to get than LPG. If LPG was the only choice for a built in, I would simply use gasoline fueled portables.