Assuming a dev is committing (ball park) 4 times per day, that's in the region of $55k/year per developer, or $16.5M/year all in. Thats not cheap...
The cost (money and more importantly time) of running tests fundamentally changes the development process.
I'm a very productive engineer on my team and I average less than 1 commit per day.
In a typical bors workflow, that holds, because you do a CI run before attempting to merge, so commits don't break the build except in the rare case that two commits that are individually okay combine in a way that breaks.
If you get rid of the individual run before merging, your combined runs will hardly ever pass, so it won't cut down on the number of runs.