10% of the price or 1000 satoshis, whichever is greater.
It is always stated that way rather than as a bare "10%", because below 10000 satoshis the 1000-satoshi minimum is more than 10%, and on the cheapest works it is a large share of the price. We would rather you saw that than discovered it.
The fee comes out of the price, not on top of it — so it is the creator who bears it, not you. The price is divided between the two payments rather than added to.
One detail worth knowing so it does not surprise you: each of the two payments also carries a small unique offset of a few satoshis, which is how the site tells one person's payment from another's. It means your total lands a few satoshis above the advertised price. Both exact amounts are shown to you before you pay.