Kalispell will be most cost effective, but about 15 minutes from downtown Whitefish and Flathead Lake, and 40 minutes from Glacier National Park. Whitefish has a lot of convenience built in. There is a reason most people who visit the area stay in Whitefish. Bigfork is a cool town right on Flathead Lake, but you would be the furthest from some other attractions. Columbia Falls is another cool city with lots of good restaurants, and you will be nearest to Glacier Park.
If at all possible, and you know you will be able to attend, Fall 2019 would be your best option to book lodging. We all know plane tickets fluctuate like the stock market, so your guess is as good as mine. Historically - waiting to the last minute is not the play.
Chris - 406.407.1853 Annie - 630.621.7788