This question cannot be answered with a single amount or percentage. For an expensive product, even a small margin percentage can result in a nice profit. For a low priced product, a high margin percentage will not produce a significant profit. A popular product in high volume may be worth selling with a small margin, and for a slow-moving product with a high margin may not make your product sell.
We recommend that when determining the margin, you should think with the head of the online store in mind: if you were the online store, how attractive and saleable would your product be with the margin you have set? Over time, the market will decide if that margin is sufficient.