The value of the product when calculating tariffs isn't necessarily the manufacturing cost, it could also be the "market value". I tried to find a source for how to calculate the value on a complete laptop but a quick search failed me.

But, for example, when bringing wine into Ontario Canada, the duty is calculated on what the Ontario Liquor Board would sell the bottle for, not on what I pay for it. This isn't the US of course, but it gives you an example that tariffs are not always paid on manufacturing costs.