Where do you want to start AC?
Have you read CASR Part 139 - Aerodromes.
You'll need an airfield that exceeds the requirements for an ALA. Put in some lights, windsocks, runway markings, etc,....you know, all the things a pilot expects to see as he tries to make a safe landing after breaking out of the cloud at 350ft AGL and a mile and a half from the threshold while still in the rain on a dull day with lots of crosswind and turbulence.(just an example boys, don't do the math) You wouldn't want him to land in your neighbours paddock now would you....or worse still take the BBQ packs and beers back to his place.
You'll need to get the runway surveyed and certified as a NON PRECISION APPROACH runway.
You'll need to survey the obstacle and terrain environment, probably out to 5km and publish some OLS data.
Getting your "landing strip" up to scratch will account for the first $4M of your budget to get your $250k aircraft into your backyard whenever you want to .....well almost....sometimes the weather is just too bad for an NPA.
Then you need to read CASR Part 173 - Instrument Flight Procedure Design.
As QSK points out you will need to approach AsA and get a quote for the design. They are the only authorised organisation allowed to design IFR procs in Australia. You will have to supply the survey data. You are probably looking at $50K per runway end just for the design work.
Then you have to have the procedures checked by CASA and flight tested. That's another $5000 per flight hour. (I hope your airfield is close to Canberra to save transit costs.) It takes about 3 hours to do flight check the terrain and obstacles to ensure they match the design data, to ensure the tracks calculated are aligned with the runway, that the distances between waypoints are accurate and to see if the desk top design is actually flyable by the average IFR Joe.
You do have a Command Instrument Rating AC? Your question and comments makes me think that you don't. Add the $30k to your budget to get one, and don't forget the GPS course too.
As your airfield is most likely private then you will have to pay Jeppesen to code your approaches into the navigation database so that your mates can fly the approaches. They are a commercial organisation and having your private airfield in the system won't sell any more Jep sets. Even the EMS heli guys have problems getting Jep to code their data without great cost.
And then you have to maintain surveillance of the obstacle environment (trees do grow too), keep the airfield and facilities up to scratch and join the NOTAM system.
If the good ol' phone company puts up a mobile phone tower nearby.....or wind farm.....or power station......or high tension power lines...or a high rise Casino, and not necessarily nearby either, because the government allows them to do so without your permission, then you will have to get AsA to redesign your approaches and flight test them again.....and then at least once every three years.
You may well have to charge your mates landing fees to share the cost.
Have you got lots of well-healed mates?
Oh, and as you get a bit wiser about airfields with instrument approaches, you can't turn around and say to CASA....."such and such a dump of an airfield has an RNAV approach, why do I have to upgrade mine?"
RNAV and conventional approaches to those dumps of airfields are getting withdrawn if the airfield does not meet the NP runway criteria, as their 3 year check comes up.
Good luck with your project.