To be honest, I think the answer to that is really dependant on the problem one is trying to solve with the app. A lot of the time, I find event apps are either poorly thought out, or cram in "high engagement features!" like a comments feed etc. (effectively trying to recreate functionality), or are trying to sell you everything and the kitchen sink.
As an example, a lot of apps point out that they have an agenda included - we have our agenda printed on the back of our badges. The problem doesn't always require software to solve, and changing the badge cost a lot less than £19k!
It's worth bearing in mind that no event app will survive one week past the event itself (or at least, it's best to assume it won't). So any value you're hoping to create for users has to either be immediate, accessible via the web, or (ideally) something they can take out of the app when they delete it.
For what it's worth, I also think that "engagement numbers" can be a fuzzy thing to calculate, and you should make sure anyone selling you an app really nails down what they mean. For example, I've run an event where 30% of users used the app. Of those people, maybe 25% actually enjoyed using it or reported getting value. And then there are the 40% people who were aware of the app, but deliberately decided they didn't want a digital experience (Without installing it / checking it out at all). It comes back to exactly what problems you're trying to solve, and what results would be meaningful towards that.
< dismounts soapbox >