I can't tell you off hand what the cheapest medical in melbourne would be
however i did mine recently in perth to renew expired class 2 was $130 and took around 45 minutes was mostly Q & A, a urine diabeties test and a few movement excercises also eye test and colour Ishara eye excercises.
The CASA fee ,for rubber stamping your results and issuing the certificate is
$ 65 depending on the sugury this can be paid to the DAME or " within a reasonable time frame " to CASA however they won't pull out the all elusive stamp without the fee being paid.
A class 1 medical depending on variables would be quite a lot more pricey especially for a initial issue, plus its valid for 1 year only.
It would be nice of CASA if they could get you to do a class 1 yet allow you to then be down graded to class 2 after 12 months until the class 1 is re validated as far as i know they don't offer this option.