How much does one need to earn in order to skip National Service?

There is no definite answer. But rumours....

if you earn high income like 8k a month or more, and you are not in a leadership position.. it is more likely they will not call you back for National Service after your ORD date.

But if you want to totally skip NS (even from enlisting), not sure what kind of medical conditions you want to find out to be certified unfit for NS.