I have imported (an exported) a fair few containers in my time. Import duty, VAT, customs charges, inspection charges, storage, fuel (I could go on) are very real costs and all go on top of the shipping cost from where ever you ship from. In my case eastern europe and the far east mostly.
Clearly the more stuff you have in a countainer the lower the unit cost of all the above will be. I used to ship 40ft containers of fabric around, so light, a bit bulky but you can get a lot in so my unit cost was less than £1. But clearly a different situation, as from a shipping perspective less to go wrong in transit.
Take a bass, in a box, in a container then the unit cost will spiral upwards to a significantly higher value. Not that up to date on shipping costs (it used to cost me around USD1,500 for a 40ft container from China) so can't speculate but I suspect the unit cost would be around £30-£40 for a container stuffed full of instruments. I could be a bit out here as I said.
Also to note that the US have a sales tax that varies from state to state which is analagous to our VAT. If you are exporting you knock that off and check charged at the point of entry to the EU at the rate prevailing.