It's however much you are willing to pay for the game. Done.
Seriously. One of the beautiful things about economics and capitalism is this principle of encapsulating the value of something with a price. Different people look for different virtues in an object. You might think time played is the correct measurement but a long game that goes bad in the end might be less valuable to me than a good, short and fun game. It's all relative but we can all get to common unit of worth through by stating the price.
If after $30 for the game and playing it, you regretted it, then it was worth less than $30. If you were happy about it, then it was worth more than $30 to you. What the game can sell for in a free market is what it is worth. That's the beauty of a free market. It's not just an exchange for goods and services, it's a good information discovery tool.
Anyone can make an omelet with eggs. The trick is to make one with none.