A standalone event logo project usually costs between €1,500 and €4,500.
The variation comes down to three things. The number of concepts you receive, the number of feedback rounds and whether a full visual identity system is included. Think of a colour palette, typography and an icon set.
Within a concept project the logo is usually part of the total package and is not listed as a separate item. For a single party €1,500 is often enough. For an anniversary or brand activation €3,000 to €4,500 is more realistic.