The answer to this question depends on a couple of variables. Obviously, what are you paying for your propane bottles/pellets?

Now, if you are able to get your pellets for below $1.5 per lb and based on the typical efficiency of an 'average pellet grill' a pellet grill can be cheaper than a gas grill to run.

We have a BBQ pellet database on our website homepage where you can see/compare the price points of the various BBQ pellet brands/flavors. We've also calculated the $ per lb costs.

Now I will note, that if you purchase a pellet grill that features twin-wall insulated construction, you can pay a little more for the pellets per lb and still be cheaper than purchasing propane as the pellet grill is more efficient and uses fewer pellets. This is also true of ceramic pellet grills.