Just tally up how long it takes to shop, cook at home, and clean up, and multiply it with the rate at $dayjob. I am pretty sure for most people on Hacker News the figure will be significantly higher than at a restaurant, and we would have to adopt a similar working style to bring the cost down, like buying and preparing in bulk, focusing on a few dishes, cooking for multiple people at once, etc.

I am not exactly a fan of this kind of comparison unless one can actually freely allocate one's time, but it is illustrative.