Hi Rocki,
You don't run them by pressure so much as by setting the O2 flow so that it bubbles through the wort and just breaks the surface of the wort. Any more and you are wasting the O2 to the atmosphere. So more of a visual adjustment.
Postsage is $12.50, anywhere in Oz.
On how many uses you get... I will come back to you on that. I am pretty sure MHB posted that here a while ago, will see if I can find it. The bottles last for ages, I am still on my first and been using it for about 18 months. But will come back to you.
Cheers