Sounds pretty cheap, relative to 300 engineers.
The cost (money and more importantly time) of running tests fundamentally changes the development process.
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.
I'm a very productive engineer on my team and I average less than 1 commit per day.