In reply to Captainsnowpack:
A slightly odd question this, as I would expect anyone who is at the point in the industry to offer courses of this nature would probably have a vague idea of how it works, but I will try and answer none the less.
If you offer open courses which require a minimum number of participants to be economically viable to run then it's up to you if you go ahead and run at a loss/low day rate for yourself or cancel.
Sometimes it might be worth running for two clients, when ideally you would have 4, as it's fairly easy to retain customers and have them book onto other courses in the future.
Sometimes it might not be worth your while to run them eg. a whole weekend for a single client and £200.
Remember that if you are engaging other freelance staff to work on courses they will expect to be paid regardless of how many people you get booked on. If you cancel the booking they will expect you to honour their booking and pay.
At the end of the day, it's up to you how you run your business and how much of a gamble you want to take. Look at the prices other companies charge and don't devalue the industry by trying to undercut them. It's not a race to the bottom. Offer high quality tuition and charge what you think is a fair rate accordingly.
Personally, I don't think open courses represent good value to the customer. Whilst it's attractive to book a weekend course at low price, the reality of being grouped with 5 other random people of very different skills/abilities/aspirations means that the instructor is never going to be able to give a client the absolute best experience. I think bespoke courses for individual clients or small groups offers better value for money even if the actual cost per day is higher.