The payoff for using a copy editor/proofreader is always worth the investment. Not only will it help you present an impeccable image and maintain an excellent reputation, but it will also increase reader confidence.
Although some computer programs may be able to highlight spelling and certain other errors, only human editors and proofreaders can professionally recognize word usage and context in the way the author intended it to be used. For instance, if you used the word roll when you meant to use the word role, a computer program may not determine that to be an error—possibly an embarrassing error at that!
Nothing can replace the human eye when you want to produce excellent, error-free work.