Forecasting freemium revenue?

Toying with the idea of a freemium-like model, and trying to wrap my head around revenue forecasting.

To expand upon "freemium-like", unlike monthly 'PRO' subscription plans that are a set value, we're considering charging for add-on features to our platform, i.e. $5 to add X feature, $2 to add Y feature.

I'm drawing a blank, but can anyone else think of other platforms that monetize this way? Mobile gaming apps come to mind, but I'm looking more along the lines of a productivity tool, etc.

Seems like the only way to predict revenue in this manner is using historical user adoption rates of said features, or benchmarking from other companies with similar services/models?