Hi there,
I'm not an engineering student but given that no one has replied I thought I'd provide my insight! My GIRLFRIEND just finished her first year in her Master of Professional Engineering. My backround is business/economics/accounting/arts(before I dropped it), and I'm at Caulfield Monash, starting 4th year, on honours scholarship, going to do commerce honours in economics.
First thing I must adress is that your question has many subjective elements. For example, what do you mean by "worth"? Is it "worth" to do a Master of Engineering to satisfy your mathematical and scientific curiosity? Perhaps. Given that you are still in highschool, I think you might be (objectively) a little naive on how difficult a Master of Engineering can be. Completing it with a 50% average is not difficult, but the only reason you do a Master of Engineering in the first place is to get a high overall mark. A masters with a 50% average in advanced engineering fields is almost useless.
Now, to answer your specific questions.
1. What difference will a Master of Eng make relative to Bachelors?
A) A master of engineering can be viewed in two ways. The first, with work experience. The second, without work experience. Internships and placements in engineering are INCREDIBLY important. A master degree with a decent WAM on it's own can be very dull to employers. A Masters with a decent WAM PLUS work experience is infinitely more attractive. Obviously, you know this, but it is important to maintain high marks throughout your undergrad years to obtain the best possible internships. After that, doing a Masters is "optional". I put those quotation marks there intentionally, because very often higher post-grad degrees are a necessary pre-req to various types of employment. For example, I'm hoping to end up at an eventual economic position. The ones at the government (e.g. treasury, RBA, e.t.c) often require Masters or Honours at the very minimum to even apply! And the private ones (e.g. Frontier economics) hold the same pre-reqs. I don't know what it's like specificially for engineering, but I imagine that the top firms will require the same degree qualifications.
2. Is graduate salary higher with a Masters degree in Australia? Commerce?
A) YES. As is the case for all post-grad qualifications, you typically tend to earn a premium above regular degree holders. This is the case for most professions in most countries really... it depends on various factors though, ofcourse. For example, how popular is the position that you are applying for? I can tell you with certainty that economic honours graduates are very low in numbers - and as such the starting salary is incredibly high! 65-70k+ for most starting positions at good fed-gov programs.
My girlfriend's course is very business related. She does a lot of statistics and business planning. I don't know whether commerce is a pre-req for this program, but she did commerce/engineering (econometrics major). If you plan on going into more scientific related positions where you will be largley working on product developments and theoretical based work (e.g. maybe electrical engineering?) then obviously commerce would be less attractive than more mathematical or theortical programs. If you plan on going into business-type engineering fields (e.g. civil engineering?) it might be more relevant.
3. Is Commerce/Engin less difficult? --> If I'm not mistaken, engineering only comprises of 1 year (1/4) of the double degree program in com/engin. Obviously, this means that you will do fewer engineering units and hence may not be required to do as many mathematical ones. I know, however, that if you're planning to go into honours or masters, you will almost certainly have some difficult pre-reqs. My girlfriend only did 1 year of engineering but even so I think virtually all of her electives were, if I recall correctly, 3rd year engineering subjects. So yes, it is "less difficult". But again, if you are not looking for difficulty, mathematically or otherwise, in an honours engineering program, then you've gone to the wrong course buddy!
Hope I was helpful.