Ph.D

It will depend on the subject you choose to do Ph.D in. In most of the institutes, the eligibility criteria to gain admission to a Ph.D in Management is Master's degree with 55% marks. In some subjects, doing a Masters in Philosophy (M.Phil) is a prerequisite to start the Ph.D. For some prestigious universities, it is required to qualify All India Level Examination such as National Eligibility Test for Lectureship (NET) conducted by University Grants Commission (UGC), Graduate Aptitude Test in Engineering (GATE), and Indian Council of Medical Research (ICMR).
Fee differentiates from college to college.

After your M.Tech, your entry cadre will be that of Assistant Professor. You can make a career progression to Associate Professor and Professor later on with experience if you have a Ph.D.

Past year's cut-off is listed: https://www.entrancecorner.com/jobs/cbse-net-cut-off/

Hi,
It depends on which field you want to do your research.
In addition, your topic for the Ph.D should not necessarily have a connection with your Bachelors or Masters studies. It can be a different, as long as you know you can deal with the research and coursework.

Eligibility criteria to seek admission in Ph.D degree in mathematics is minimum. B.Tech candidates from any engineering discipline are also eligible to seek admission in Ph.D.
You are not eligible for seeking admission in Ph.D in mathematics with B.Com or M.Com degrees. PG Diploma is not to M.Sc mathematics, therefore, after PG Diploma in mathematics, you will not be eligible for seeking admission in Ph.D.