Hey, I'm auditioning for a band and they want me to record my vocals over their instrumental demo. How do I do this at as little cost as possible?
Go to a rehearsal studio tbh. Just say you wanna record some vocals quickly for a demo, probably only charge the cost of the room for an hour + a small fee for setting up. They might even mix it in to the track for you.

It'll show some dedication to doing a good job, which any band will appreciate.

It'll cost you less to do a good job there, as opposed to doing an average job at home. Just know the songs really well, get in and bosh it in an hour.

Good luck dude!
Yes, agreed. For what you'd pay for a half-way decent mic and preamp and interface, you could easily do an hour or two even in a pro studio. Assuming you are a half-way serious singer (you know... based on the fact you're auditioning for something that requires a serious demo), you should be able to go in, set up, do three takes, make a couple of minor edits, quick mix, and be gone in an hour.

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.