It's not just the parts you have to figure in, it's the cost in time and space.
Remember they're businesses and the time and space in their workshops are both valuable, while they're doing an easy job for one person they could be doing a more profitable job for someone else, so a lot of them tend to have a minimum time cost, for example a job might only take five minutes to do, but their labour fee will be a minimum of one hour because they have to arrange their jobs in such a way that if they run into a problem and take longer than they should it won't effect other work,
Also note that while yo know one part is faulty and needs replacing, that will not be the only part that needs to be replaced, for example in your case when replacing the pressure hose they will also need to change the o-rings and drain and replace the power steering fluid, these things while small also add to the cost, then on top of that they will need to spend some time testing and making sure there's no leaks etc.
However, in your case, in my opinion you're seriously being ripped off (unless there are other outlying issues with the vehicle that increase the cost), the part itself as you say would be around $50au, the minor parts would probably come to maybe $40au as an overestimate, and considering for a decent mechanic the job would only take ten minutes to half an hour with a decent amount of testing and checking tops (it's a matter of unbolting two nuts, draining the fluid, putting new o-rings on the end of the new part, bolting the two nuts in and filling the fluid up again, a simple job), you should only really be charged for an hour of his time at the most, so say a generous $80 for that, making the total roughly $170au in my opinion (although that is based on converting to pounds and comparing prices i pay in the UK, average prices might be different where you are, specifically in more remote areas where there is a shortage of qualified mechanics, plus i'm basing it on my experience of power steering which is limited to one model of car so might be more difficult on your car than i realise).
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