To summarize a few past answers, the closest thing in ancient rome to a cheap junk food was probably roasted chickpeas. We know that this was considered a lower-class snack because in plays and other literature to be deemed a ciceris emptor, a "buyer of chickpeas" was a classist slur, implying something like calling someone a "Doritos-munching slob" would today.

This question has come up a couple of times before in various guises. There's always going to be more to say, but while you are waiting for fresh responses, you might like to check out these earlier threads:

How was Roman street food served and eaten given that disposable materials were much less available?, with u/XenophonTheAthenian

I'm sitting here eating chips. Did the people of the Roman empire have an equivalent of such snack foods?, with u/samadhii

What kinds of street food would of been available in Ancient Rome?, with u/JDHoare