What kevinr says. At some point, you start reading what you expect to read rather than what is actually on the page--including typos with red lines under them. You basically cannot publish something significant without having another set of eyes on it at a minimum. A decent copy editor will also help fix up some things (in my case excessive wordiness). Tools like spellcheck help but only to a degree. Also as kevinr says, basic copyediting is pretty cheap. I paid about $400 for my last book.
