Hi,
Thanks for using our plugins. I can help out here.
If you do not want to add a price you could just leave it, but that is up to you. I have no information that this will hurt or help SEO in any way.
Google Recommends you have it, but they do not required it as not all events have tickets and that is what I believe it looks to be intended for. They consider it a warning not an error in the structured data too.
It is possible with a customization to change the output for events without any costs, but we do not have a feature to do that on it’s own right now.
If you think that is something you would like as a feature I encourage you to visit our User Voice Page and either upvote an existing request or make a new request there.
http://tribe.uservoice.com/
Let me know if you have any follow up questions.
Thanks