Of course you could just buy brand new plates from here
http://www.myplates.com.au
these are the official suppliers now that the rta has offloaded the job
STG17 is available for $100 upfront and a $50 per year fee ... so basicaly all those other prices are complete bollox
I think the special ones like "SAINTS" or "DRAGONS" are a bit different & as far as prices go, it's just a matter of finding the right person who's prepared to pay 4,5 or even 6 figures for certain plates. Whoever has them probably wants to keep them, but I suppose it's just testing the market out of curiosity.