Prep time is a real concern, but a vastly more complicated one than comparing dollar amounts. Even if you manage to figure out a fixed dollar cost of prep time, most people aren't in a position to directly trade hours between food prep and making money.
Anyway, as long as we're throwing random cost factors into the air, fast food has much bigger labor, utilities, and real estate costs that their food prices have to cover. And individuals can do pretty well buying bulk if they put in the effort. But we talked about effort already. It's definitely not going to be simple if you want to fully quantify all the tradeoffs, but if you just count dollars, well, there's a pretty wide spread of effort levels where you can beat fast food.
Anyway, as long as we're throwing random cost factors into the air, fast food has much bigger labor, utilities, and real estate costs that their food prices have to cover. And individuals can do pretty well buying bulk if they put in the effort. But we talked about effort already. It's definitely not going to be simple if you want to fully quantify all the tradeoffs, but if you just count dollars, well, there's a pretty wide spread of effort levels where you can beat fast food.