It's certainly not cheap - no matter what way you go.
You can apply for the partner visa directly but you want to be sure you meet all the requirements and have enough evidence. So you have to have been together for 12 months (that's a requirement, doesn't matter how mature you are or how solid you feel your relationship it). The only way to waive that 12 month requirement is to get married or register your relationship with a state that allows it.
But getting married isn't enough in itself. So you will still need lots of SOLID evidence to show you meet all 4 evidence categories for a good amount of time:
Joint finances
social evidence (usually the easiest category to meet)
nature of household
nature of commitment to each other
If you live apart more than you live together in the past 12 months and you are applying from a high risk country then a PMV would be better suited to you. Coming from a high risk country you would really need to ensure that your evidence is air tight which is hard to do if you don't live together.
Also, remember that if you apply for an 820/801 FROM a PMV the cost of the 820/801 is reduced. So you costs would be as follows (in AUD):
PMV - $4630.0
820/801 - $1145.00
TOTAL - $5775.00
