As a freelance writing professional you want to make sure that the product you release to any client (large or small) is up to your standards of quality. No matter what the cost it’s worth it to deliver a professional product.
If you accepted a writing assignment only to later realize you’re spending more time than the assignment is worth, your choices are 1) take the loss and deliver the professional product or 2) speak to the client to see if you can negotiate an increase in price and deliver a professional product whether the client agrees to the increase or not. In my book, there’s no third option.
One shoddy piece of work can follow you through your freelance writing career. We now live in the Internet age where things follow you for the rest of your life and longer (I’m sure Michael Richards wishes he could burn that unflattering YouTube display).
Don’t burn your bridges. If you’re not proud of what you’re about to release, don’t release it. You don’t want it to come back to bite you in the rear later.
One small financial loss in your freelance writing career now is a great learning experience for how to negotiate your rate for future assignments. Learn from your mistake and keep on writing.