I've imported several doubles from the UK, Africa, and Australia. The costs are:
paperwork on that side, permits, fees, etc.
international shipping by air with associated fees to the airline
brokerage fee this side
import fee
storage and hold fee
import paperwork charge
customs duty
shipping to my FFL
FFL's fees
Average was between 2-3000 USD from the seller to my door.
Cal
For auctions in England, I came up with a simple formula. Whatever you bid in pounds, double it and that’s your landed cost in dollars.