I asked my agent the same thing and in a nutshell, they can definitely come on a parent visa and will have permanent residency when they land as well so all state benefits/obligations will apply as if you are an Australian citizen. Basic criteria is as follows:
More than 50% of your children must have PR and must have been residing in the country for minimum of two years to qualify for parent visa.
So if you are one of two kids and have been in Aus on PR for minimum two years they will qualify and costs are as Renny said, in the region of R1mill. Processing time is approximately 13 months. Can be more though but generally around there.
I just want to tell you this though. Even though the money is probably the biggest kicker and might put you off you have to view it objectively.
Many private services we pay for here in SA like medical, car insurance, security services, etc, will be costs you won't incur in Aus.
I did a very basic calculation and based on average medical aid expenses, retirement savings and insurance fees that are not really necessary or at least not at the levels we pay here, the visa will pay for itself from retirement age before the parents reach age 75. I worked on a very middle class family income and related average expenditure on above mentioned goods and services. So if you're the kind of person that make a move like this happen and your parents are also in the same category you should be able to do it with relative ease. Of course, you would unfortunately need that sort of capital to begin with though.
Medical conditions are reviewed on a case by case basis and they are all unique so unfortunately no clear answers there. However, the diabetes itself does not mean automatic disqualification. As Renny said it will be based on all sorts of factors.
I hope this helps