It seems you've left some things out of your math.
You've neglected to consider the costs of putting on the seminar (the 5 staff needed for your example are expensive to pay, transport, feed and house and the venue also requires payment for services), time outside the seminar (prep and revisions, grading), taxes (something you have to be self-employed to appreciate the whole of) and insurance.
Use your imagination as the above is not anywhere near comprehensive, and opportunity costs even aren't considered at all.
But you do know that "expensive" isn't equal to cost and cost isn't equal to value, right?
You have to make that sort of decision on your own.
If you're feeling lucky you could always find your way to WFAC, pay the $8 day rate and hope you get your money's worth.