Commerce is a very broad field and salaries can range from $50k p.a. to $1,000,000+ p.a.
Some fields in commerce are more difficult than others, like actuary, some are super competitive in terms of employment like investment banking, some are relatively easy like accounting, some are mainly academic and won't get you employed like economics, etc.
Doing a degree in commerce doesn't necessarily mean that you will have a clear career path because if you hate it and are not passionate about it, you will not get get a high paying job.
Melbourne, in my opinion, is the better university for commerce. But your choice should be determined by distance. If you live one hour from unimelb and 20 minutes from Monash, then you should definitely go to Monash.
Since you say you are good at maths, have you considered engineering or computer science? They too have a clear career path if you are good enough at them.