If your room isn't treated, your vocals will sound thin and hollow. This is the biggest problem in home recording. You don't need a thousand dollars; you just need some blankets and a little creativity.
The Goal: Absorbing Reflections
When you speak, sound waves hit your walls and bounce back into the mic. This creates a "boxy" sound. To fix this, you need to absorb those waves before they reach a hard surface.
1. The Blanket Booth
The most effective budget solution is to hang thick moving blankets or heavy duvets in a "U" shape around the singer and the microphone.
2. Recording in a Closet
A closet full of clothes is a natural acoustic absorber. The irregular shapes and soft fabrics of the clothing will break up and absorb sound waves perfectly.
3. Floor and Ceiling
If you have hard floors, put down a rug. If you have a low ceiling, hang a blanket above the mic.
Treating your room is the single most important gear "upgrade" you can make.