I have a friend who's ran a seasonal fireworks shop on the side for the past two years and he claims it is close to what you just said. However, thats if you follow the rrp, which is often quite high. he normally sells things 20% below rrp.
3-400% mark up would be about right. Some makes give a artifically high rrp makeing easy discounting possiable and people thinking there getting a good deal. Storage regs and insurance etc all add to the costs, its only a short season, a couple of times a year at most really.
