The permit fees for an event permit are set by the local council and therefore vary widely. Rough guideline:
Small events (private, 50-200 people): often €0-€250 in fees. Mid-sized events (200-1,000 people, public): €250-€1,000. Large events (1,000+ people): €1,000-€5,000+, depending on the deployment of police, first aid and traffic measures.
On top of the permit fees, the local council sometimes also charges for additional costs. Think of supervision, enforcement or the placement of traffic barriers. Always check this in advance so you're not caught out.
Costs for Buma/Stemra, a sound consultant or a safety plan come on top of this and are invoiced separately from the council fees.