I did the numbers about 2 years ago.
NG has right at 50% less BTU's than diesel. That means almost 50% less mileage than diesel. :E
If you have your own pumping station (several more thousand $$$'s) and cheat the tax man to save the road tax and put on a lot of miles, and there is a big difference between NG and diesel, you might come out on the plus side of things if you keep your truck for 15 to 20 years +. :E I'd rather spend my money on a really good investment like the lotto. :B
BTW where are all the people that argued no power loss with NG last time around? :B If your still out there, you need to talk to Chevy because they haven't figured out how the bring the power back up when using NG. :B