In my 30 years of doing shows for a living I have to say that nothing good ever came out of a free show.
They are always set up wrong.
The promo for the show is always about them and very little said about you except that YOU are doing the show FREE.
The day of the show is always crazy and runs late, so your 20 minute free thing turns into 2 hours.
They will promise you the sun, moon, and stars but they never come thru.
If the free show is a charity or organization that you want to support or that you believe in, then do the show.
But what do I know?