Our CI runners live on a box in the corner of the office and their only operating cost is my time.
Paying someone for CI compute seems insane. The load is so variable that you never know if your monthly bill will be zero or several hundred/thousand dollars. I especially don't want my employees to consider that each and every push costs the company a nonzero amount of money. CI should be totally free and unrestricted. If a new employee has a really bad day and fires off a hundred CI runs (as we all have), I don't want to explain to accounting why there's an enormous spike in the bill.
It costs us a couple of my salaried hours a month to maintain our on-site infra. Far, far less than our present AWS bill. Most months it needs no attention. It just sits there and does its job. Hell, it's even solar powered.