going on a 1 year holiday to perth, australia. What should i know about budget and finances?

As others have said, you can't stay for 12 months on the visa available to you, which is a 3-month tourist one.

On the subject of finances etc, surely your "long distance boyfriend" would be the person to discuss this with.

US citizens can visit Australia as a tourist for up to 90 days by applying for an ETA:

https://www.eta.homeaffairs.gov.au/ETAS3/etas

There are no boyfriend/girlfriend visas. You cannot simply stay in Australia for a year.  

Really you are just moving in for a year?

ONLY if you have a visa.

BUDGET? Shall we all guess if you expect nightly five star gourmet meals or you

will cook at home?

Are you on a work holiday type of thing?

Sorry NO ONE can guess what job you might have.

Similar expenses to the USA not identical.

Suggest you not spend it as fast as you get paid. You need to save for your trip home.

Getting married or whatever is some other immigration issue.

There is no girl friend visa.

Australia Immigration decides if you can enter and stay a year NOT YOU.

BEFORE you can enter Australia you are expected to show a reasonable method of funding your expenses while in Australia.

Live in Perth? Seriously do you think everyone has Velcro on their shoes so they do not fall off the planet?

It is a nice place but is isolated from most of the population in the country.

https://www.youtube.com/watch?v=4xdm96OC3vQ

WARNING getting married or breeding does not give either MOVE IN rights in either direction. The child can move back and forth NOT THE parent.

If that happens CHILD has TWO parents that each can claim things for example custody. You make an agreement or the courts figure something out.

You are most likely thinking of something like this. READ what you can and cannot do. It is not guaranteed.

https://immi.homeaffairs.gov.au/visas/getting-a-vi...

Do you have your visa yet?