This almost made me laugh out loud. Exactly how many kettles does the author think sells each year? Multiply by models and again by manufacturers, and you're talking millions of kettles. Even if you consider that one model will last multiple years, manufacturers don't buy supply batches that last for years. Some industries go in the extreme other direction - the local car industry got its stock-on-shelf lifetime down to 48 minutes from part arriving to part being screwed into car.
Similarly, retail markup is generally 20-50% of the final cost. If you think this is too high (it really isn't) then I suggest you use your new formula to undercut the existing market and disrupt away. Other significant costs are transport and wholesalers (who aren't always the manufacturers). Kettles are bulky items that consume a lot of space in storage and transport. On top of all this, every party involved in the transaction is entitled to make some profit, and not just work for costs. There's also regulatory compliance, things like the CE mark.
Looking at an item and stating that the price shouldn't be more than parts + amortised kickback to the designers is naievity, plain and simple.