Do you not effectively put a dollar value on things you do for entertainment / personal satisfaction / fulfillment? Pick any two activities, and you can probably identify a dollar amount (which might be infinite) that would induce you to do one rather than the other.

So let's say you're playing a video game, and someone asks you to mow their lawn. How much money would they have to offer you to induce you to do so? That's the marginal dollar value of that video game over mowing their lawn.

Or let's say you're playing a video game, and you need to mow your own lawn, but you don't want to. How much would you pay someone else to mow it so that you can keep playing your game?

Of course, those two amounts would be different because you probably feel differently about mowing your own lawn than about mowing someone else's. The difference between the two should (if you're being consistent, which humans seldom are) be how much would someone have to pay you to mow their lawn instead of your own.