You dont/cant convert a 457 into PR.
You need to qualify and apply the same way anyone else would. You can apply when you want but if you wait for 2yrs having worked in the same employment you don't need to do the skills assessment.
As shel stated there is no connection between 457 and PR.
Skills Assesment : xyz (ACS for IT is 400 AUD)
IELTS : 330 AUD per person in Australia. Better do it in india. chances are higher and cost is low
Application form : 2960 AUD
PCC (Indian) : 48 AUD per person. Can do from india as well
Police clearence (AUS - if you have stayed in AUS) - 45 AUD per person
Medicals (If done in AUS) : 317 AUD per person