To add or edit the price of an Extension, start by clicking Prices on your Projects page:

This opens the Products page where all Extensions in your project are listed. Click the Extension you want to add/edit the price for:

If a price has already been set for the Extension, you'll be able to view and edit it right here:

If a price has not yet been set for the Extension, click the Add button to do so:

As the next step, you specify the price in terms of plan, currency, discount etc. before saving the information: