I'll guess that price was somewhere east of the Cascades, not west. And not in Leavenworth either. But picking up friends at the Calgary Airport almost two weeks ago, I was surprised to find gas much cheaper than here in the metro Seattle area: CDN$0.879/liter at a Calgary Costco, converting to US$2.506/gal. Even the tourist trap prices in Banfff were no higher than Seattle's current average of $3.37/g, though we do have plenty of sub-$3 choices for anyone who looks at the price before pumping. As expected, nothing along the BC leg of the trip was comparable.
North Bend ($2.559/gal) and it was $2.83/gal at Burlington. Alberta has the lowest gas prices. Edmonton is the lowest in the country (I mean the refineries are right there). In B.C., you'll want to fill up in Kamloops. (and then in the Fraser Valley or I guess just cross the border if you use Sumas)
The lowest Gasbuddy posting for North Bend is currently $2.97, though I do remember some lower prices in the region around the time of your trip. But that Burlington price still has plenty of company. For our trips that far north, we start with a full-ish tank, top up at Sumas, then refill at (or near, depending on convenience and lines) the Kamloops Costco. Though our return this time was a different route, to visit the Myra Canyon section of the Kettle Valley rail-trail. We also used this trip to re-enter the U.S. at a different location, the north end of Ross Lake, for a nice day hike to Lake Hozomeen. For those unfamiliar, this is a place where one can drive into the U.S. with no essentially border control or ICE or Border Protection presence most of the time, just an infrequently staffed National Park Service Ranger Station to issue walk-up backcountry and boating permits. But that road doesn't connect to anything, so would-be smugglers face a very long and rough backcountry hike to the next road. It seems likely that the spooks have some form of monitoring hidden along the route.
