Paid event but Pricing Options can not be required but they must chose a fee
My non-code way of (almost) solving this is to include a zero price "no-selection" option such as "I not attending the full event"  or  "I have selected a different workshop" in each price field and make each price field compulsory.

My experience has been that people forget to tick anything, rather than deliberately choose not to make a selection, so forcing people to make a selection for each field has worked for us.

I should point out that I have only done it with 3 sets of price options, and I didn't get any flack about making people tick so many boxes.  I don't know how people would have reacted if they had been forced to make 6 "selections".

The "almost" in the first sentence is because each year we still got one or two out of 1000 people who chose the "no-selection" option in every set. I set up a smart group for people with a registration value of zero and sent them a standard email alerting them to that fact.