“The plant quoted me a lower number. Why would I pay more?”
Because that number is not the number you pay
A plant quotes FOB. Its responsibility ends when the machine is over the ship’s rail in China. Ocean freight, marine insurance, the pre-shipment conformity certificate your customs demands, terminal charges, duty, clearance, inland haulage — all of that is still ahead of you, and none of it is in the figure you were given.
We quote CIF or DDP: one figure to your port, or to your yard, with the certificate and the export documents inside it. It is a bigger number on the page and usually a smaller one at the end, because nothing has been left out to make it look attractive.
If the landed total is genuinely cheaper to assemble yourself, buy direct. We would rather lose that order than win it by hiding the cost of the parts nobody quoted you for.