If you're 18, that means you've got college coming up. That's going to mean a lot less time that you can devote to your BPs anyway.
I'm not saying a coral glow is a bad idea. It just may not be the best timing. If you could get one this year, breed it and sell the babies before you started college, maybe that's a decent idea. But, once you get into college, you're going to want to have some spending money sitting around. Trust me, beer and girls ain't cheap.
As far as mom goes, I think the best thing might be to compromise. Maybe she'd like you to sell some of your other snakes first before you got a coral glow. You know, just ask her what she thinks you ought to do and listen to her advice. Personally, I wish I'd saved the money I spent on crap like video games or play-by-mail wrestling games or hell, drugs... I wish I had that money now so that I could spend it on a coral glow.
Anyway, best of luck whichever direction it all goes for you.