I have written this post with the aim of answering this question for non-lawyers. If you're a solo or small law firm and you stumble upon this post, I think it's safe to say I don't know the answer to this question for you.
I feel strongly that there is no reason that you should pay for legal research if you're not a lawyer. Most legal research that you'll find is cases and caselaw, and people go to law school in part to learn how to use that material and turn it into something that makes sense to other lawyers. If you don't have that training, it's going to take you an awfully long time to figure out how to turn these cases into something intelligible. Besides, you may even find that what you need is available online. See a future post for details on that.
Now, you can spend hundreds of dollars a month on legal research if you wanted (Lexis, Westlaw, anyone?) but there are cheaper options (LoisLaw and VersusLaw can both be had for under $40/month). This is not to say that they're going to get you what you want, even at these reduced prices. If you need a brief, or something that uses legal authority to persuade a court that you're right on an issue, legal research isn't going to help you unless you know how to use it.