That's a great smoker - buy it.
I'm about a month away from delivery on a YS640 with competition cart, these things are back-ordered at about 3 months currently.
Two big factors on pellet use - ambient temp and cooking temp. For smoking butts, a YS480 (probably the model he has, and the one I've used) at 250deg will use about 10 lbs at an ambient temp of 60-80 deg, assuming you're cooking for somewhere around 12 hours. more butts will increase consumption, as will lower ambient. You will use less pellet weight than charcoal, because of the more efficient fuel delivery and smaller burn footprint. The firebox on these things is quite small, and the Yoder's are built with a lot of mass, so keeping temp steady requires less fuel overall, and you're not cycling through as much. It's kind of like an old AC air conditioner unit vs the new DC systems. The old AC unit kicks on, blows full throttle, and then shuts off and the temp climbs until the next cycle (charcoal). The DC systems are basically made to run all the time at different speeds to keep a consistent temp - a good pellet smoker does the same thing by monitoring temp and constantly adjusting pellet flow to keep it within a degree or two of the set temperature.
