Green’s Dictionary of Slang

(US teen) money.

1982		M. Pond Valley Girl’s Guide to Life 52: Billies—Like money.
1985		‘Jennifer Blowdryer’ Modern English 6: billys (n): Cash. That awful green stuff you need to get into a show, you know, you parents have it.
2002		‘Valley Girls’ on Paranoiafanzine 🌐 And even if you don’t have much billies, you can go hang out with your friends anyway, and try on make-up or something and check out the dudes.