Ask HN: What to do with potential partner with 100M users?

My startup provides a marketplace for digital content, we have been working with partners ( who sells the digital content to their end users ) through their own product. The model with partners is simple, when a digital content is sold, the partner and us gets a cut, and then the rest goes to the content creator.

Our cost of goods sold is mostly hosting and bandwidth. We only incur bandwidth as part of a sale ( user pays and then the download occurs ). We have been seeing good traction and recently a potential partner approached us to partner up. They have 100M MAU so working with is a no brainer.

However, there are some concerns, but mainly it's the amount of infrastructure cost of going live with this partner.

1) We have been supporting partners with our current infrastructure of around 1M MAU. So on boarding this partner could mean we're looking at 100x of our current infrastructure cost. Its a naive calculation but is the worst case scenario.

2) We offer free downloads as a marketing & promotion tool to end users as part of the end users on boarding to help them get comfortable with the new content offering in the partner's product, converting them to paying users.

3) Most partner integration takes time to warm up and we see revenue ramp up from month 3 onwards.

The potential benefit of this deal is obvious since we could potentially increase our revenue by 100x ( naive calculation again ).

A) What is a good way to determine potential cost of infrastructure? We're looking at the first 6 months of going live with this partner before revenue comes in. I've researched into high scalability's article on Instagram's cost, is there any other good references we can study to determine cost of supporting this scale of users?

B) What other concerns should we be looking into other than infrastructure cost?