5c increments for options?

Let’s say bid: .05 / ask: .10
– what determines what my actual cost will be when I pay the ask price for a contract? Or is it random because it executes anywhere from .07-.10.
– on the flip side, if i then want to sell the contract for a profit at a later date, how do I list to maximize my gains? Let’s say a week later, bid: .15 / ask: .20.

Thank you