To really do your rifle justice, chuck your Nikko Sterling in the bin, spend some of your aussie dollars
and invest in some quality optics, you will not be dissapointed with the difference when sight up through a top notch scope. You mentioned Leupold Mounts, well their scopes are excellent, they will stay with you when you upgrade your rifles, otherwise you might end up with the situation where you upgrade your rifle and HAVE to upgrade your scope.
That 17mm (whats that about 11/16") group is very promising and carefull handloads with MKs will bring you 5 shot 1/2" groups and again a better scope will help you realise your rifles potential.
$220 is indeed steep for a 20 moa base even compared with what we here in Blighty pay for them, about £100 is the going rate, plus about the same for a set of tactical rings.
You can easily spend one and half times the price of your rifle on optics and mounts.
Try these links, i dont know if they will export to Australia but i dont see why not, if not maybe some of your countryman on this forum can help you.
All of these products are available from Brownells and they will export to Australia.
"I meant to shoot the pike but the duck got in the way"