If your company is making $1 mil per employee per year, then 10% is 100k. Even at 500k employee or lesseer numbers it's almost always better to buy the $1000/month tool (break even is a measly $108k revenue per employee per year)
It's not just about cost, it's about having the control, stability, and autonomy of on-prem. Plus you can probably repurpose that compute when employees are out of the office.
What if that tool is something you can run on prem, and over time make the investment back?
It's not so simple.