We buy "fast food" maybe 3x/year, on road trips or the like. I have a bunch of Subway gift cards from unknown events, and haven't spent them in years. Maybe I will treat the teens to Subway this week, as they each have a bunch of AP exams & they love Subway. We're not big sandwich eaters, so rarely have sandwich fixings at our house. Perhaps that's part of the Subway appeal.
If we're on a road trip (for us,most likely driving to Lake Tahoe for skiing), we typically gas up at a Costco, and buy the teens lunch at the food court. It's under $5 for both of them. Neither DH & I eat it, and we'll have had a late brunch before we leave.
I did just buy Chipotle while on a hiking trip, and my mom & I split a burrito bowl. Reasonably priced, I suppose, for the quality of the ingredients & the fact we could share. Additionally, we went to In & Out on the way to the airport, and bought lunch for two for $12. Pretty reasonable. The last time I was at In & Out was a year or so ago, when my parents were visiting.
Similar to Michael in ABQ, my teens have their own money (earned through summer jobs) that they use for meals out when we aren't there. They definitely purchase fast food, and make regular comments about how expensive it is. So, they are aware & it's all part of the process of them learning about priorities, budgeting, spending money, etc.
Anyway, we're not big diners out anyway, but typically if we're eating out, it's for a special occasion & thus definitely not fast food.