Anywhere from 20% to 50%, depending on the mood of the custom guy!
But legally; you're supposed to pay around just 3% for laptops, tablets and desktops:
So... It's best to take a printout of the aforementioned document and if he charges you anything beyond 5%, show him the document and ask him to give you an explanation, which is something he won't be able to provide so he'll likely start babbling and throwing bullshit right in your face; in which case ask him to show his work I.D and NIC and threaten to press charges and sue.
Yes, you heard me. SUE!
I'm sure that's enough pressure to make most custom officials come back to their senses. Most people are scared of lawyers; including me!
For better effects; tell him how this will get him suspended from duty. But with that being said; stay 100% calm and confident throughout the conversation, even intimidate him a bit but don't take any personal shots at him and don't raise your voice, otherwise things might get ugly!
It's a bit tricky, but entirely doable, my friend.
Hope it helps!