To evaluate the effectiveness of event promotion, you need to compare the cost of the advertising campaign and the number of applications, contacts or conversions to participation.
If tickets were sold for the event, you should calculate how much it cost to sell a ticket from advertising. Evaluate the ROAS (Return On Ad Spend) metric : divide the income from sales and attraction through advertising by the costs of advertising. You can calculate the inverse indicator DRR (Share of advertising expenses) : divide the costs of promotion by the income from it. The ratio will show how much was spent to earn one ruble.
It is more difficult to measure the image and information organize your finances with mint effects. They can be assessed by indirect metrics such as LTV (Lifetime Value), i.e. the profit from one client over the entire period of interaction. If LTV grows after an advertising campaign, it was probably effective.
To assess how the promotion of the event affected its image, conduct a survey among the participants. Ask how they learned about the event and what influenced their decision to participate. If the press and bloggers were invited to the event, you can count the mentions of the event in blogs and the media.
This way, you will be able to assess how effective the advertising campaign was and whether it is worth sticking to a similar concept in promoting future events.