Plan for roughly double the single knee price, $6,000 to $12,000 all in at Australian rates, staged as two operations a few months apart with the worse knee first. Some clinics discount the second procedure; it is always worth asking directly.
The more valuable question is whether both knees genuinely need surgery. Grades commonly differ between knees, and a grade 3 that is causing lameness does not sentence its grade 1 partner to the same operation. Plenty of dogs have one knee fixed and the other monitored for years.
Before committing to a two knee surgical plan, have the imaging and grades reviewed independently. Halving a surgical plan is the single biggest saving available in this whole area, and it is only ever legitimate when the second knee truly does not need it, which is exactly what an independent surgeon can tell you.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.