That depends on your target audience and reach goals. For a regional corporate event with 200 guests, you can already get results. Count on 500 to 1,500 euros in social ads. For a national conference, count on 2,500 to 7,500 euros.
Start small and measure what works. Save the largest part of your budget for the final two weeks. That's when most registrations convert.
Read more in our article on using social media for your corporate event.