When I interviewed Jake Jabs 10 years ago, he shared with me that he wrote his own copy. He did it better than anyone else. It has always been one of his keys to success. It also led him to doing his own commercials because nobody could sell furniture as well as Jake.
The TV ads became so important, they ended up creating a studio in the companies headquarters.
Copywriting is a skill that most of us simply don't posses. When it comes to hiring an expert, this is one area I recommend investing. Can it get expensive? Yes. Consider the main issue - the copywriter is going to have to spend a great deal of time understanding your product/service.
Can you help them by providing the info they need in a concise manner? Yes.