A dynamic community and active following sounds like you may have the opportunity to drive revenue with the merchandise. That being said, my advise would be to start with the right margins so you don't have to adjust your retails as you scale. At the very least, start with a 50 pt margin. I suggest a 60pt margin - you will encounter costs you weren't expecting and this will help you maintain profitability. The correct way to calculate this would be the following example: let's say you can have a mug produced for $5. Your retail price should be about $12. Take $5 / .4 = $12.50 then round down to $12, leaving you with a 58 pt margin. If you have ideas about wholesaling, then you would have to adjust this strategy slightly to allow for aligned retails between you and your retail customers. Normally adding a 35 pt margin to the cost price and then calculating the 60 pt margin on top of that would suffice.

Kamal's answer is pretty spot on, but I would like to add that it's important to stipulate that their shares vest over a period of 3-4 years. A one year cliff is also appropriate.