Zach Brooks pocketed $1,000 this month blogging about the cheap lunches he discovers around midtown Manhattan — $10 or less, preferably greasy, and if he’s lucky, served from a truck. The site, MidtownLunch.com, is just a year and a half old and gets only about 2,000 readers daily, but it’s already earning him enough each month for a weekend trip to the Caribbean.