So just how much is this garden going to cost to build?

This was always going to be a difficult question to answer.
In the UK it's comparatively easy. There is an annual publication called Spon's. In that you will find a price schedule for most jobs that you are likely to come across in landscaping, e.g. laying turf, laying stone paving, building walls etc etc.
Most contractors will refer to it when costing a job. Some will charge more than the quoted price (if they are in demand and/or very good at what they do), some will charge less (if they need the work). Location is sometimes a factor, but by and large the quotes that you receive will be about 10% either side of what a designer could sit down and work out themselves as the actual cost.
That simply does not happen in WA.
For a start there is no "Spons" to refer to. There is no recognised fee structure here. As I said to my first client, price is as variable as the wind and I couldn't possibly try to guess what a design would cost to build.
The second reason for this is due to the local economy. For those of you who are from further afield than Australia, WA is based on mining. Mining has been booming for the last 10 years on the back of the Chinese expansion. As a consequence, anyone who fancied a life in the outback, in exchange for a pretty healthy wage packet, could "go work in the mines". Tradies - electricians, carpenters, brickies, cooks have all been snapped up by the mining companies. As a consequence, those that are left (not everyone likes the idea of 12 hour days and 2 weeks on 2 weeks off - often living in a prefab) are often either very expensive or not very good.
So, there is a major skills shortage once the mines have mopped up the best.
Some landscapers can pretty much charge what they like, because they have a surfeit of work because the work that they do is of a good quality. Others, I have on good assurance, are simply crap at what they do.

Landscaping, unlike building, is unregulated here. Anyone can do it without any qualifications, licences or experience. And don't think being a member of a trade organisation is any recommendation. It isn't.
This is also true in the UK amongst tradesmen. Some that are not very good, often push to get into a Trade Organisation, hoping that a badge on their adverts and publicity will be taken as a guarantee of quality. Unfortunately it doesn't always work like that, but at least you have someone to complain to when things go wrong. Whether or not anything is done about it if you do complain is another matter.

This was confirmed to me when I put my first design out to tender. These quotes were not even for a complete garden, but for about 2/3rds of the build.
The first company didn't even bother submitting a quote. I chased them by mobile, landline and email and I never got a reply!
Company no. 2 quoted $50,000. A little over what I was expecting, which was $40,000.
The last company submitted just under $100,000!

With such a disparity in quotes, what chance has a designer got in costing a design for a client before the design is completed and put out to tender?

As a newcomer, I can't help feeling that the people here are getting ripped off by some tradesman. But then I don't have the mentality of a boom economy where most people have money (in some cases an awful lot of money!) and don't mind so much about the cost of things, because they can afford it.
In the boom period to 2007/8 the quality of some building work was appalling. I have a friend whose house is falling apart around him because the quality of labour and the quality control was so poor due to lack of skilled labour.

So the moral, please don't ask me to guesstimate the cost of a project. I can give you a rough idea but, when contractors charge what they think they can get away with, it really is anybody's guess.

So where do I go from here?

Setting up my own contracting company, I think.
At least that way I can dictate budgets as I design, rather than have them dictated to me after the design is completed!!