Not pistols, pencils. Virtually free vs. not at all free. It's cool that cost is no matter to you, but kindly understand that there really is a difference in cost, and it matters to some.
So I did the math, tablespoon to fill airloc, 15 milliliters to a tablespoon, 750 milliliters to a bottle, 750 divided by 15 is 50, 50 divided by 6$ is 14 cents, so it costs me 14 cents to fill my airloc with vodka. Now cost does matter to me, a lot. But 14 cents for piece of mind, common now. Having something that WILL NOT loose it's sanitation properties, or foam out and leave my airloc dry, is worth 14 cents to me.
Now, starsan after the end of brew day is free. But what if you come home and it's dry? Now you have to blend up another batch. I used to keep it pre mixed, and a lot of people do, but I'm a perinoid freak, and don't trust it. So I don't keep it pre mixed any more.
Once again, it comes down to personal preference. I prefer vodka, you prefer starsan. Now, if I really wanted to, I could drive half a block to my mother in laws (or walk) and fill my airloc from her VERY big liqueur cabinet for free, and she would not care. Now it's free
but, I always have vodka on hand, because I use it to soak fruit in before pitching into my ciders (I always have a cider going) so I wouldn't go over there because I'm somewhat lazy. So I guess I could fill my airloc from my soaked fruit jar if I really needed to as well.