I see it less as a matter of certainty and more of a matter of situation.
Your first sentence is dealing with a hypothetical situation.
I wouldn't sell my house for two million dollars.
That means that, even if someone were to offer me two million dollars for my house, I wouldn't accept the offer. No one is making such an offer, but I'm saying that I wouldn't accept it.
The version with "won't" may be talking about something more concrete.
I won't sell my house for two million dollars.
The tone of this sentence makes is sounds like a real offer is either in the works or on the table, and the person is saying they will not accept it.
Going back to your sentences:
1) My daily time with Sandra is something I wouldn't trade for anything.
2) My daily time with Sandra is something I won't trade for anything.
Both sentences mean that you value your time with Sarah, but in the second, it sounds like someone is trying to get you to give up that time – perhaps by rearranging your schedule – and you don't plan to budge. But I wouldn't regard the first sentence as "less certain."