It does not cost any gas fee when you make an offer on an item that is not on sale on OpenSea. If your offer is accepted by the owner, the gas fee is going to be payed by him or her. However, if it is your first time making an offer using WETH, you need to approve OpenSea to trade WETH by completing a one-time free transaction. This one-time free transaction costs gas, but it is only one time. It does not cost gas to cancel a bid too.

Another cool thing about making offers, not just directly buying an item on sales list, is that you can make number of offers even if you have don't have enough WETHs to buy all of them. For example, if you have 0.1 WETH, you can make offers on multiple items with 0.1 WETH, and only the first item that is going to accepted is gonna become yours. I hope this helps.