High Volume API can be added to Zendesk Professional and Enterprise plans. The cost is €25/license/month (minimum purchase equal to 10 licenses, so €250)
Why add Light Agents?
Lift your API usage limits. Our top plan includes 700 requests per minute (RPM.) This add-on increases that to 2500 RPM to handle your larger scale projects.
For a deeper dive, check out this one-sheeter about our High Volume API