It's not about how much one could eat, but what's the optimum amount to stay fit. Too less can lead to undernourishment and too much to obesity. Either way you aren't going to stay fit.

Likewise, there's no such established rule to decide a reasonable profit margin on merchandise. You should start with calculating your cost, both direct as well as indirect, and then decide how much mark-up above the cost do you intend to earn. Again, there could be some expenses that may arise at future date, more like a business expense than direct expense related to the product/merchandise, and you may need to consider them as well.

As a matter of fact, in my decade of experience working with businesses across the globe, I have found pricing strategy to be the less priority item on list. And, that's where a business makes the first level of mistake towards suffocating the brand value and equity. Do let me know if there's something I could help you out with. I am just a clarity away for further clarity. All the best!!