How much is your time worth per hour, considering your skill/talent/experience, etc? Let's say you decide $10 (I'm just using that because it's a nice round number and will make the math easier LOL). The workshop itself is 7 hours. But you can't just charge $70.
You also have to consider how many hours of prep goes into a workshop - making the sample, preparing step-by-step demo pieces, writing and printing any handouts, etc. Do you have costs involved in promoting the workshop, special supplies, renting a venue? What about travel time and gas costs to and from the workshop?
Assuming you are doing this as a business and are self-employed, now you need to add in an amount for self-employment taxes and other overhead. All things the casual observer doesn't think about - "Oh, Susie charges $200 for a 7 hour workshop - that's over $28 per hour - wish I made that kind of money!" I usually figure I'm actually earning only about 1/3 of whatever the hourly rate is - so now we're down to about $9/hour. (Just like the plumber doesn't take home the $60/hour he charges to fix your toilet - a lot of it goes to overhead for the business.)
You can also look online on what other teachers are charging, but keep in mind the 'famous' ones or ones teaching their own patterns/techniques can charge quite a bit more.