Sorry if my comment came across harshly, I didn't intend it to.
By expected value, I mean the price as you'd calculate it with a risk-neutral valuation based on some model of the underlying security.
For example, if you have a model that says an option is worth $4, and it's selling for $2, you should buy it if you're confident in your model. If you can do this repeatedly on a bunch of independent options you'll make money in the long run (assuming your model is correct and you're placing a large enough number of bets relative to the probability of making money on an individual option).
I learned this with a combination of practical experience, self-study, and coursework.