I see what you're doing, and you're completely correct. It's expensive to make meals solo.
But it's an uphill battle and you won't win.
Nobody cares that buying new ingredients to make tacos for a party of 1 is expensive.
If they aren't doing it yet, they will soon be telling you that you aren't holding it right, wherein:
You should enjoy leftovers. You should embrace and cherish the idea that electing to have some tacos one night dictates your meal choices for the days that follow.
That extra chopped onion that is all stinky and sulfurous by day 2? The meat that isn't ever going to be remotely the same again? The tomatoes that are reverting to slime? All that extra lettuce? That fresh cilantro that doesn't have any other culinary use in your chosen diet? Buck up and eat it, or shut up.
People are broadly incapable of having non-disingenuous conversations about the cost of preparing fresh food in small quantities.
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Tacos for 2? Or 3? Or 8? Or 16! That's really easy and efficient to make for a one-time meal at home. It scales up very well, indeed. It's a great way to feed a bunch of people relatively inexpensively.
For a person cooking for themselves who isn't inclined to eat the same thing for multiple days in short succession, tacos get do kind of expensive -- expensive enough that buying them hot, fresh, and pre-made becomes very attractive.
(My personal favorite is when they tell me that I should try having different taste and just change my preference for the foods that I eat instead of complain about the price. Or when they tell me that I'm a very poor meal planner just because I want only one burrito: FFS.
Sometimes a burrito is just a burrito, and that's OK.)