Obviously, if you're going to do voiceovers, you need a controlled sound space in order to get the job done. Many people spend tons of money building an elaborate and complicated setup - and it will work - but you can do almost the same thing for a lot less.
I've been doing studio recording, radio and voiceover work for decades, and I've learned a few tricks. It comes down to a few clever choices and a little thought, which can make all the difference.
You'll notice in my photos below that the entire room is not treated, because it's simply not necessary. What you need is the immediate space around the microphone - maybe 3-4 feet - to be well treated against reflections. The rest you can do with much more inexpensive methods, or not at all. For example, the wall opposite the opening in my booth is covered with a comforter, just something to knock down the most pesky reflections.
A quick PS. I've already had several people tell me, "Are you insane? Why would you show your studio? Won't people think you're some low-end operation if they see it's just a makeshift space?" First of all, it's not haphazard; it's very specifically thought out. Could it be more beautiful? Of course, and the only person who's going to feel better about me, is me.
Honestly, what the space looks like shouldn't matter; the results you get should speak volumes. And a client that you work with online will only care about the results as well.