The sell price isn't based on a fixed markup on top of the cost of the goods, it is based on maximising profit, which is determined by both the price and the quantity sold:
for example, for a product that costs the retailer $10:
[TD="bgcolor: #FFC443, align: center"]Price[/TD]
[TD="bgcolor: #FFC443, align: center"]GP per Unit[/TD]
[TD="bgcolor: #FFC443, align: center"]Quantity Sold[/TD]
[TD="bgcolor: #FFC443, align: center"]Gross Profit[/TD]
[TD="align: right"]$15[/TD]
[TD="align: right"]$5[/TD]
[TD="align: right"]125[/TD]
[TD="align: right"]$625[/TD]
[TD="align: right"]
$20
[/TD]
[TD="align: right"]
$10
[/TD]
[TD="align: right"]
110
[/TD]
[TD="align: right"]
$1,100
[/TD]
[TD="align: right"]$25[/TD]
[TD="align: right"]$15[/TD]
[TD="align: right"]65[/TD]
[TD="align: right"]$975[/TD]
[TD="align: right"]$30[/TD]
[TD="align: right"]$20[/TD]
[TD="align: right"]50[/TD]
[TD="align: right"]$1,000[/TD]
[TD="align: right"]$35[/TD]
[TD="align: right"]$25[/TD]
[TD="align: right"]40[/TD]
[TD="align: right"]$1,000[/TD]
[TD="align: right"]$40[/TD]
[TD="align: right"]$30[/TD]
[TD="align: right"]25[/TD]
[TD="align: right"]$750[/TD]
The retailer will choose the price that they estimate will yield them the highest profit for their total sales.
That's all very well for a retailer, who at least has a good handle on most of his costs. But even there, the pricing decision is much more strongly dependant on the sales estimate at each price point; slightly different sales estimates could lead to radically different pricing - for example if at $30 he can actually sell 56 units, then a 50% price increase will yield higher profits, all else being equal. Costs are not all that important; if the cost estimate of $10 is out by +/- $1, the optimum price point will likely remain unchanged.
Sales estimates are hard to judge; so many managers concentrate on the cost element of the equation, which is easier to get a grip on; but that is a mistake - the best results are to be had by improving the quality of the sales forecasts, despite the difficulty of that task.
For manufacturers, the cost of a unit is also difficult to determine; there are armies of industrial accountants employed to work out what proportion of the cost of everything (eg. the wages for a clerk in human resources; the cost of waste disposal; the electricity bill; the water and sewerage charges; rent for the buildings, etc.) should be assigned to each of the products coming out of a factory gate - and even with a huge (and costly) effort, the final guess at the total cost contained in a final product is pretty woolly.
The story becomes even murkier when we look at marketing and advertising costs; because those costs are, by their nature, going to vary as some (probably complex) function of the volume that can be sold at a given price - if your advertising agent is good at his job, you might be able to add a few percent to costs, and get an increase in sales volume despite increasing your prices.
At the end of the day, in my experience, small and medium sized businesses set prices based on the boss's gut feelings, and modify them over time using the same technique. If they hit the magic point where profits are maximised, it is more by luck than judgement; although in mature markets looking at history of sales at different price-points can give you an edge in making the decision to raise, lower or keep prices on hold (notice that this historical technique doesn't require any knowledge of costs to implement, either).
Large enterprises, in contrast, hire an army of planners, cost accountants, and sales estimators, to try to get the right answers. Having done all three jobs at one time or another, I can say from experience that this replaces the boss's 'gut feel' for where the optimum pricing point lies with a more technical and mathematical approach, which incorporates as its strongest influence, the 'gut feel' of the planners, cost accountants, and sales estimators.
So in summary, prices are not set at the level that maximises profits, so much as they are set at a level that a bunch of guys guess might probably if they are lucky and with a following wind be close to the point where profits are maximised. Economics isn't called 'the dismal science' for nothing.
If physics was on a similar basis, the speed of light would have been determined by Léon Foucault saying "Well, it has to be more than 10
5m.s
-1; and it seems to be less than 10
12m.s
-1; so lets plump for 5x10
8m.s
-1 for now, and see how we go for a few months".
Estimates of error are rare in economics; not because such estimates are particularly hard to make, but because anyone making such estimates with any degree of accuracy or honesty would scare the pants off everyone, and probably get fired.