Gunbroker fees
I've only sold one item (parts kit) on GB so far. Just to give an idea, the parts I sold for $926 had a final value fee of $52.80. Roughly 6%. The final value fee will vary a little depending upon what listing options you choose as a seller. There is a compliance fee (1% of item price) and taxes that also have to be paid, but those two are added to the final price of the sale, so bidder is paying that to the seller and, then seller gets invoiced for those as well and pays that back to GB.
To answer your question $1700 + $17 (compliance fee) + $119 state tax (estimated at 7%) = $1836 final price at closing. 6% of final price is $110 and some change. The $110 + $17 + $119 would all get paid back to GB.