Variable cost models for Customer Onboarding

Hello GGR Community.

I lead our Customer Onboarding function. My team delivers implementation services to get our customers live and adopted on our products. Once a customer is adopted, they are formally transitioned to our Customer Success team.  Like our Customer Success team, our Onboarding team is a cost center rather than a profit center.

I am challenged by variable inputs into Onboarding.  Volume of new customers/projects sold can vary significantly from month-to-month and quarter-to-quarter. I need a cost structure that can vary as sales and inputs vary. 

Our Customer Success leader suggested that this forum would be a great place to hear different perspectives on variable costs.

I would love to hear about your good or bad experience with variable cost approaches. Thanks in advance for your thoughts!