The Mexican food truck that comes once or twice a week to the south side of 96th St., just west of Broadway. I guess you could get tacos etc. there, but Carol and I used to buy the whole dinner plate (cheap yet good) -- say, pork in an adobo sauce over rice and beans -- for less than $10.
Also excellent: the Halal truck on Sixth Avenue near West Fourth (or is it Third?) Street.
Meryl, sorry it's taken me so long to get back to you. Slight problem with my hard drive. My former hard drive. Anyway, the halal turck I mentioned is Sammay's Halal Food on Sixth Avenue, just about West Fourth St. (east side of the avenue). Sammay's makes terrific kebab over rice for $5 and even more terrific chapli kebab (the chopped, sausage-like meat) over rice (same price). There's a veggie platter ($5) that's varied and good, and the falafel sandwich ($3) is both delicious and a steal. FYI, you can also get a hot dog at Sammay's for a buck fifty, but why would you?