Most of the clinics I have been to charge an auditing fee if the clinician is a little bit of a big name. Some are free to audit if a member of sponsoring group to a wide range of prices. Many offer lunch if they offer it to riders, so the cost is a bit more. Last one I audited was $25 including drinks and snack.
I have learned to check on a few things before paying to audit. Is there a sound system? Sometimes the instructor talks quietly on a headset or walks out to the middle of the ring and you cant hear anything! Then I check the clinicians style if possible. There are some that I would ride with but wouldn’t audit as they do not explain much.
If you wish to accomodate auditors, please think of these things as well as seating or flat spots to put the chairs they bring. And try not to have the farrier working right in the doorway, drowning out the sound, It is really nice when there is somebody from the barn there to encourage the gossipers to take it outside.