Three separate amounts can apply to a Solana purchase or sale using OpenSea.
The OpenSea fee, which comes out of the seller's proceeds.
Creator earnings, if the collection has them. The buyer pays these on top of the item price.
The Solana network fee, plus a refundable account deposit when you create a listing or an offer. Both are paid in SOL.
As a buyer, you pay the item price plus creator earnings, and you pay the network fee. As a seller, you receive the item price minus the OpenSea fee. When you accept an offer, the OpenSea fee and creator earnings come out of the offer amount, so the buyer pays exactly what they offered.