I would agree, researching your industry is going to be the key. It seems to me the markups in retail are huge, but there`s lots of middlemen who provide the convenience of local shopping, timely delivery, some sort of guarantee, etc.
Something like Corn Flakes are probably only 5 cents (at cost) for the actual product, but the retail packaging is the expensive part. For estimation purposes, a good guess is the price doubles every time somebody buys it, and does some sort of value-add like packaging, distribution, etc. So the box of Corn Flakes is $4 or $5 at the retail level. You can probably work backwords by halves to see how many levels of people touched it, right back to the farmer.
If your idea is a material item (not software), you are competing with competitively similar products made overseas. Your low start-up volumes and high cost of local materials, labor, etc. Make it a difficult deal for you, because your example buyer can get similar things from China for less than you can produce it.
Do lots of research before you tackle any direct-to-retailer deal. You may be better off with your own website, with a retail price that allows wholesalers to buy from you at a 50% discount, if that is possible with your industry and product.
I hope this helps!
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Hamilton Jones
wilham@hamiltonjones.com