They were paid off their rank and time in service. You also got a few specialty bonuses for special jobs/assignments. All military pay charts are available online for viewing for free because it's a public service.

http://m.military.com/benefits/military-pay/charts/historical-military-pay-rates.html

Compared to today, I can tell you that the grunts who aren't NCOs or officers make around $2,000/month in combat.