I sauteed some garlic in olive oil until it was golden brown and then added canned tomatoes leftover from last night's fish recipe, dried hot red chilies, and, just for funsies, some anchovies and basil (neither of which were part of the recipe, but I wanted to use them up.) The sauce stayed on the heat until it thickened up.
Meanwhile, I cooked the penne according to the directions on the box in heavily salted water. Once the pasta was ready, I strained it, mixed it in with the sauce, and voila. Dinner.
penne -- $2.29 (with a little more than half the box remaining)
Total Cost of Penne all'Arrabiata: $2.29
(with one serving leftover)