Hard to say Michelle. First level diagnostics indicated multiple module failures on my Jeep and (higher price), but I was told a more thorough check will be needed. It may well be that more than one part is needed for your problem.
Here is the part ($US) that was replaced on my Jeep. Chrysler part # 68145593AF
Module. Shaft lock - steering column, steering column. Export. 2015 Jeep Cherokee. Genuine Jeep Part - 68145593AF (68145593AB, 68145593AC, 68145593AD, 68145593AE). Ships from Mopar Parts Webstore, Orlando FL
I think you are being ripped off price wise. It sounds opportunistic. You may want to confirm the part number.
Generally you expect a car part to attract a 40% to 50% margin - so cost=AU$350, sell = AU$700. Anything more is a rip-off.
No way should it take a whole day. Sounds like your mechanic has to learn and figure out how to do the job on your dime. My case labour is estimated 4 hours.
That being said, I am not a mechanic and can only go from my past ability with car mechanics and what my experience was with this problem.
Parts availability may be a problem. I was lucky.
If you are stranded car-wise, it may pay you to take thing into your own hands. Call around the country to all Jeep Dealers and ask about the part number (I don't know the part # of the 'receiver hub'). You may get lucky. There has to be a part somewhere in Oz!
One option is to use your research and approach your mechanic. Hopefully, if you have a good relationship with him, he will see reason and compromise.
The other choice is to speak to your local Jeep Dealer (if you have one) and see if it will be worth getting them to do the job. I am usually against taking my cars to authorised dealers as I am not the trusting sort. In my case, it worked out because they had the part and the know-how to do the job. I doubt the average mechanic has the know-how.
Hope that helps