I'd say about 2 gallons of gas for that size mound. To me, nothing works better than gasoline for killing yellow jackets of fireants. (Given it's far enough away from your home or another building)
Take a piece of rebar, pipe or shovel handle, about 5-6 feet long, stick it through the middle of the mound, then jab it again next to that...repeat and keep repeating. When you have holes down to where the pipe hit's hard dirt (the mound dirt will be soft for a good couple/three feet down), pour your gas all over the top of the mound and into the holes, then pour you a trail 20-30 feet away from the mound. Let it soak in for a good minute or two, then light the gas trail (preferably with a match, as you can "toss" it).
Stand back, drink a beer and enjoy your victory.
Repeat in a couple days if necessary, but, I've never had enough survivors to rebuild the mound.
With Yellow Jackets, don't light the gas, just pour a gallon or two down the hole and walk away. The burn method doesn't work with them, I don't know why. I found that out last year, had to burn them out 3 times before I finally got them. Oh yeah, be sure and do this at NIGHT when you are gassing yellow jackets...they aren't active at night and they will all be in the hole.
Podunk