Do I pay anything for a collab stay?

The stay is free in exchange for content. Extras on-site are separate.

Free Stay Collabs

The stay itself is free in exchange for the agreed content. That is the whole point of a collab: your content is the payment.

Anything extra you add on-site, like meals, a spa session, or paid activities, is between you and the property unless the deal specifically includes it. The deal page tells you what is covered.