Locally, a head of lettuce ranges from $1.25-ish in the summer for a good full crisp head of lettuce to $4 in the winter for a little wilted tasteless thing, and the lettuce from the "eat locally" CSA program is even more. If they can do good fresh lettuce for a couple dollars year round, they should be able to make some money.

A 40' container probably shouldn't even require one full time staff to harvest, it could be feasible for a grocery store to operate one of these. Dedicate a few staff hours per day, sacrifice a few parking spaces.