I think it is very important that "someone" look over a book, even if one can't afford an editor. As others have pointed out, a writer can get so close to his/her writing that problems that might be very obvious to someone else are not seen.
If nothing else, get some friends who are willing to point out what they see as possible problems to read the manuscript.
I am currently a "reader" for a book. Not exactly the same as an editor, but still. A friend is writing a history of a photographer active primarily during America's Great Depression. She got three people to read her manuscript. I was asked to because I helped catalog the collection of the photographer's negatives. So I was familiar with his work and the background history.
I read the manuscript and found what I thought were several problems in terms of structure, etc. I honestly did not feel as if I had been terribly insightful, but she reworked entire sections as a result and says they are improved. Will re-read again shortly.
Okay, obviously all of this does not apply to self-pubbed fiction, but at least get some friends who are not afraid to point out what they see as flaws. They shouldn't expect money, although they would probably appreciate getting a copy of the final product.
And yes, I think grammar/syntax/spelling does matter--at least to some of us. When I see such errors, I feel a writer is being sloppy. Of course I am a librarian, so I am overly detail-oriented at times.