If you are usa based 2500 and under should be duty free 18000 would be at least 3% duty = 500
I must add that the threshold is PER_SHIPMENT being processed by FedEx/Customs so if you try to get around that by making multiple smaller orders you better be sure they ship out at least a week or more apart so they are not processed at the same time.
Also be aware that when US Customs applies Duty they also notify the State where the goods are shipped to. If your state collects sales tax on out-of-state/internet purchases the state will bill you for taxes due. Been there - done that...
Or perhaps have several different people place orders and ship to their different addresses?
Then again, since you are planning making sizeable income: Register as a business. Then most things including cost of the miners, electric, etc, become deductions from reported income. Even better -- ref the IRS Section 179 new equipment purchase deductions
http://www.section179.org/section_179_deduction.html
