I have bought Suzuki parts from USA and generically the cost of the part is lower but the import cost is only slightly less than the cost of the part.
eBay normally gives the, p and p, cost next to the item. If you want a rough idea. Go to eBay bang in Suzuki GS ( loads of parts in USA so a good bench mark) and scan through the results. The pricing is very uniform and so allows an accurate guesstimate of costs.
Everything I’ve had from the US has had 20% added by the carrier (royal mail usually) they send the bill for the 20% that has to be paid before they will deliver. They would also add an admin fee for the charging.
If you argue, (I was mostly receiving items that i was going to do artwork on (WW1/2 Vietnam helmets etc) and then send back to the owner) they just wouldn’t deliver and send it straight back to the US. A helmet worth several hundreds of pounds would add significantly to the cost of doing the artwork.
I had to tell any customer that I would have to add 20% to the cost on the value of the item (the value is written on the import sticker by the sender).
A guy asked if he could send a Catalina’s door to me to paint, I told him it would probably be cheaper to pay for me to travel to him with my brushes.