For quick maths - take the price of your item, then add the shipping, then add Vat to all of that. Import tax itself depends on the item. IIRC guitars are about 4%
Bascially get the item and shipping and add 25% and you'll be close enough.
Then also take into account whether it's an item that might have warranty issues down the line, as you are likely to be knackered on that front if it's from the US, or indeed outside of the EU.