That article gives you the 'ingredients' list, and rough material cost on the last page, so you could do some sums and it should give you a ballpark cost for materials only. Labour costs is where it will get expensive though, depending on the geographical area some carpenters/builders can charge $60-ish an hour up to over $100. You can look at teeing up an apprentice to do the work (with their mentors permission), which can cut the cost in half, or look at local (decent) handymen. That can bring the cost down to $35 an hour, $40p/h or there abouts (again, depends on location, prices vary wildly throughout Australia).
When you are getting any quotes for work, always get a minimum of three so you can compare prices. Some will under-price and leave you with shoddy work, or charges down the line, and some can over-price because they don't want to take the job. Always aim for the middle price, and ensure it lists all the work to be done.
If the coop plan is pretty much exactly what you are after, I'd be inclined to print it off and give it to the people you want quotes from. That way half the work is done for them and they should be able to give you a price pretty quickly. You really do need to work out what your budget is, and not go over it. Decide an amount you feel is acceptable for you to spend, and go from there.
The other option is to try and DIY it. It looks pretty simple, so borrowing or hiring tools for a weekend and you could knock it up pretty easily. This is coming from a person who -with no prior experience- has since totally renovated and extended a house, built a carport, built a front fence, and my chicken coop, aquaponics, among other things. All it takes is a bit of research and preparation! Well, and time...time is the one thing we all are short on, lol (which is why my front fence is still without gates, but shh, lets ignore that, haha).