1) Is it for a professional setting or a social setting? The more that money's on the line, the more valuable it may be to you to invest in a speech writer. Although there are plenty of reasons to hire for a social speech (read on!), people in social settings are pulling for you. They're generally not expecting or demanding too much. So unless you personally feel that you need to wow them, you may feel fine with writing your own speech for social settings.
A couple exceptions come to mind on social speeches: wedding speeches by the best man or maid of honor -- you've been selected as the most important friend or family member on someone's most important night. A good time to be funny and heartfelt without any awkwardness or inappropriate stories. A professional writer can help. Similarly with a sizable gathering for a retirement, where you're one of the few who will help to recognize someone's life work ... maybe time for a good social speech. In short, any social speech that represents a key moment in someone's life would be a good time to invest in help if you're ever going to.
2) Do you have the money? Maybe an obvious point, but for some people, money is easy to come by while time is not. You're going to have to put in one of the two. A speech writer won't eliminate your need to put in time, reviewing drafts and offering input. But he can help you avoid staring at a blank computer screen or revising that sentence for the 500th time.
But for those who have more time than money to invest, try writing the speech yourself if it's not "mission critical." You may find that you're happy with what you write, or that you're able to hire someone to just smooth it out, saving some money along the way.
3) What other value do you get from the help? Besides reducing the time you need to put in, does a professionally written speech help to put your mind at ease when you're presenting? If so, that's an invaluable asset for all those who dread getting up to speak. Or maybe you've been giving the same annual speech for years and you need a fresh message.
This is similar to assessing the value for any kind of professional help. Do you spend hours learning from YouTube how to fix that home appliance, ordering parts, then doing it yourself and hoping you get it right? Or do you cough up a couple hundred bucks to have it done by a pro, preferring to save your time for other things while improving odds that you'll have things right in the end? There's not one right answer for this. Sometimes we do things ourselves. We have to pick and choose when.
4) Can you get your message across through a purchased speech? It doesn't help you to buy a speech if the writer does a poor job of getting your words or message into written form. So if you're going to buy a speech, make sure that the "intake" process is thorough. This may be done by phone or by you filling out a form, but it needs to let you convey your thoughts and extract as many details as possible. An experienced writer will know how to ask the right questions, or will have a thorough form developed after having written for hundreds of clients. He's also learned how to write in a variety of voices to match a client's tone.
Given this point, you might ask a service how they gather your information before you invest in a speech with them. I have a thorough form for wedding speeches. For other clients, I write a series of questions for them to answer by e-mail or talk with them by phone, as they prefer.
I hope this gives you a few points to ponder when you're considering whether to buy a speech or stick with writing your own. If it's ever time to go with a professional, I welcome you to reach out and let me know how I can help!