The expensive part of an opportunity may be the better opportunity you become unavailable for.

The cost of a decent opportunity is not the effort it takes. It is that you are occupied when the better one arrives, and you will usually never know it came. That is why saying yes to everything reasonable is not a neutral strategy. It fills your capacity with the merely acceptable.

Before accepting something that is fine but not exciting, ask what you would want to be free for. If the answer is something specific, protect the room for it. Availability is part of what makes good things possible.