Is it worth it to pay a professional editor before submitting to publishers?
It depends on how clean your skills are.
First, you always, always, always need several content readers to go through your manuscript before you send it in. These should be fellow writers, your critique group and/or others who are well read and who have no emotional investment in protecting your fragile psyche. These readers should do what we call a “content edit” to evaluate your story, characterization, flow, style, plot, etc. to help you find inconsistencies and plot holes. They’ll probably also pick up a good portion of your grammar mistakes and typos.
Second, you always, always, always need someone to do a copy/line edit of your manuscript before you send it in. If you’re highly skilled in editing yourself, then you can probably get by with a proofreader who has a strong grammar background. This could also be a fellow writer or someone in your critique group.
If however, you know you have difficulties, or you don’t know anyone with the appropriate skills who’ll read it for free, then yes, hire someone. Make sure they have credentials and experience–a high level of grammar and writing skills; an idea of what is currently selling (as in, they read a lot of popular writing, as well as the classics); and happy, repeat customers who have been published.