So, if you did eco through arts, what would the additional courses be? or would there be none at all?
Absolutely no additional courses at all! My degree is Bachelor of Science (Advanced Mathematics)/Bachelor of Arts, and while there are some compulsory electives for science (e.g. 24 junior credit points [4 units] from at least two different areas of science), I didn't have any compulsory courses for B Arts, i.e. everything I do in my B Arts is directly related to economics (I chose to maximise my B Science with maths and statistics). However, under B Arts, you are allowed 60 senior credit points (10 units) under a single subject area, though that's more than you need (as you only need 24 senior economics credit points to major in economics), this means you'd have a bunch of free electives, which you can use to complete another Arts major if you like.
If you are definitely going for economics, Bachelor of Economics is the best route. In some universities such as USYD, B Arts is used as a leverage in combined degrees to major in economics (i.e. there is no Bachelor of Science/Bachelor of Economics, hence Bachelor of Science/Bachelor of Arts substitutes those economics students [since economics is often administrated under Arts/Social Sciences]). So in summary, if you choose against Bachelor of Economics to major in economics for some reason, Bachelor of Commerce often requires you to do a commerce/business major along with compulsory business units regardless, and Bachelor of Arts may put an upperbound on the number of economics units you take.
But in the end, regardless of whether you did B Arts/Commerce/Economics, you would graduate with the exact same economics major and studied the same units with the same cohort.
Good luck!