July 1 is Canada Day
July 4 is US Indenpendence Day
End of June is beginning of summer holidays in Canada and US
The world famous Stampede in Calgary is begging of July
You are doing a one way rental
Calgary is the arrival point for Banff and Rockies visit

This is super high season for car rental in Calgary already; not to mention your requirement of one way and the Calgary rental car inventory is reduced.

If you see a price you are comfortable with, book and lock it in and see if a lower price comes up.