If you trade options using market, you will buy at ask and sell at bid. But on ET we don't trade on market orders, we always use limit orders. Usually but not always, you can buy/sell at roughly mid between bid/ask so the slippages are not as bad as you said. You won't get mid if you are DOTM or DITM. With DITM you usually get less than intrinsic if you sell to close and for DOTM often the bid/ask are so wide you are better off not trade those. Good luck.