The bar I always use for "prank" is:

Something you can do to a complete stranger, and that stranger willingly acknowledges the humor value -- beyond "it was funny for other people to laugh at me" -- after the fact.

(They may not personally think it was funny, which leaves some gray area for failed/bad pranks, but they at least acknowledge that it had humor conceptually.)

>"on hand" ruler

I see what you did there.

For me, it almost never works out well to give them any words to work with, because the depth of their delusion/narcissism will just allow them to stoop to whatever level is required to give an answer consistent with their worldview.

If I were you, I'd do the 5-ish seconds but then instead of finishing with words, give an incredulous chuckle. As in, "Wow, I can't believe you really thought that was OK to say out loud" or "what you just said was so ridiculous I can't come up with any other response than laughter". Then redirect your attention immediately. That way, if they try to engage you further, you can hit them with the "are you still talking?" attitude/body language. By actually responding (with words) you're on some level giving them permission to further engage and acknowledging the validity of the conversation.

Not sure if this will help on that small of a budget, but one thing to bear in mind is that having some different seasonings (sriracha, curry powder, etc.) can make the same stuff "stretch" -- in the psychological sense -- a lot further.

EDIT: I realize the OP is about actual food, but I guess the angle here is you could buy a larger quantity of a cheaper -- but boring -- food and still keep your sanity. :)