They are priced 'all over the place' right now.
The diesel tend to command around a $20K premium over the gas.
I have the gas and have never found it lacking for power, even on I-75 in Georgia and Tennessee.
I found mine on ebay, but you can shop and do research at http://www.rv.net/
They have a large classified section and the forums there are a good place to see the strengths and weaknesses of different models and manufactures.

They only started using this chassis in the ~2004 year, so you wont find any as cheap as benspeed's Class A, but the deals are out there.