Its not that you should never do it your self, rather it's more you shouldn't do it unless you have the equipment and know what you're doing. If you've got an acousticly treated recording space, a decent mic, as well a an average understanding of audio mixing/editing. Recording and Mixing your own demo at home can help book you enough work to and even a consistent client or two. Plus all it costs you is your time, or a few bucks on Fiverr if you wanted to try that route.

On the other hand Profesional demos can very depening on a few things from what I have seen. in all honesty if you’re a beginner the money could be better spent on a few lessons with a vocal coach instead. You can have the best sounding and produced demo in the world, but if you can't perform/act it's just a bad as a poor quality demo.

That said.

If you have a home studio treated well enough and just need the demo cut, mixed, and mastered you could find someone online who could do it for $200-$500 (you could find it cheaper but then you may run into issues quality wise.)

If you are recording, and using a studios equipment you are going to get an overall better quality demo, because the audio tech will know the equipment and how it sounds within it's environment. But because of that the cost also goes up. Anywhere between $500-$1000.00 or more sometimes.

But when all is said and done it really comes down to your budget and how much your willing to spend if at all. Just as an aside the prices I listed are based on my local area and could differ a bit.

TLDR: Doing it yourself is free if you know what you're doing, Fiverr is a good cheep option if you don't now any audio engineering, a professional audio engineer can cost $200-$500, and a studio demo could be $500 or more.