This is don through market research. You look at similar things that is already in the market. You can interview people asking if they are interested in the product. You can ask experts in the field such as shop owners. You can release some information about the product and see how much interest there is in it. Or maybe the most accurate way today is to accept pre-orders, this is often what kickstarter is used for.
There are some models which attempts to gauge how the demand changes with price. And it would be lovely to know that for example increasing the price by 10% decreases sales by 50%. But these models are highly dependent on the type of product and the market.
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