Hi,
We include the offer in the structured data with our Ticketing Plugin, Events Ticket Plus.
From my understanding it is looking for more then just a price for that field, but an actual link to buy tickets online.
It is also a warning and not an error according to Google.
So we do not have fill that in in the free version. Some people may want differently, but that is not a feature we support.
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/
Cheers