I’ve been wondering something. If I were to get Unity Pro with a subscription for $75 a month, would that be until the subscription is cancelled, or until that money adds up to $1,500?
Until cancelled. The two license models are different.
On subscription you have no up front cost, but will be paying the cost every month forever.
With buying a license you only pay once. You must purchase a new license at major version upgrades, ie 4.x to 5.x