Most every synthetic 5w40 runs around $26-35 a gallon, though in some Walmarts you can still find Shell Rotella T for around $22.
I was able to pick up some 5-gallon pails of Mobil Delvac 1 5w40 for $55 each, and to pick them up locally, so it cost me $11/gallon. But that was an unusually good deal, and even then the stuff was CH-4 rated so probably 5 or more years since it left the plant. I ended up selling it for basically what it cost me to a few friends who had more use for it than I did.
At one point I had the 25 gallons of Mobil Delvac 5w40 plus another 30 quarts or so of other synthetic diesel engine oil... all for one car that only needs about 4.5 quarts every 10k miles!