Okay, so I'm going to talk about proofreading - because it's something that a lot of independent authors and small publishers neglect.
There are two myths:
1. You don't need proofreaders any more because spellcheck catches everything. I can always tell when somebody thought spellcheck catches everything. The thing is - spellcheck will catch some things. It will catch typos, it will catch reversed letters, and it will catch doubled up words. It won't catch when you typed "there" instead of "they're". Oh, and grammar check tends to produce results that read awkwardly in fiction. I'm not saying spell checkers aren't useful - they are. But they should not be the only thing.
2. You can proofread your own work. I actually saw a job ad for writers where they said "Proofreading is 50 percent of a writer's job" - so clients often think this too. Fact is? You can't. Why? Because you know what you meant. With the best will in the world, you will never produce a hundred percent clean copy without somebody else proofreading it. (And to be honest, even multiple proofreaders don't always get things to a hundred percent).
So, what should you do?
You could hire a proofreader (Hi! I'm available). If you do, this person should be the last person who looks over your book other than you before it goes to print. I even recommend doing layout first. (And I also recommend, strongly, because it caught me out last time ordering print proofs and reviewing them before you post the ebook version). Any more substantive edits will introduce new errors. Trust me, they will.
Or if you can't find room in the budget, then find a writing buddy willing to trade with you.