We were averaging around 5.7 L per 100 km for the entire trip, as per the trip computer. In the mountains (depending on the incline) we would get 8 to 10 plus litres per 100 km going up 6% to 8% grades, although some sections of the road from Invermere to Panorama and from Field to Takakkaw Falls were fairly steep. We used 2nd gear quite a lot, and 1st gear on some really steep switchbacks on the way up to Takakkaw Falls (larger vehicles, campers and buses that couldn't turn around had to go backwards up and down the switchback):
Attachment 3819
Attachment 3820
Here's a video on You Tube:
https://www.youtube.com/watch?v=PUYizM0qisA
This is one of those times we really did appreciate the Micra's tight turning circle!
Gas prices at Fairmont, Golden, Canmore and Invermere averaged around $1.15 to $1.17 per litre of regular over Canada Day Long Weekend, so a full tank would come to about $40+. For the entire round trip (about 2000+ km from Edmonton to the Kootenays & back) we probably spent about $150 or so for gas.
It was just my wife and I with our luggage so Little Guy wasn't too weighed down to keep up with highway traffic and pass slower vehicles going uphill.