If India drops Iran's oil this will be also expensive. At this point India will have to compare financial harm from obeying the White House to the financial harm from disobeying. Let alone the political damage.
Iran makes and sells oil, a product that is in high demand. The USA makes ... what?
And here is the problem. As soon as a major country tells the USA to mind its own business, the USA loses weight on the political arena. But perhaps that's the goal.
Were I in India’s shoes, I might tell the US that if they go through with sanctions, India will make their business climate extremely receptive to businesses currently operating out of the US. That India would set up special incentives for such businesses that move operations to India so their employees receive healthcare (the good hospitals in India’s system are quite good, and very inexpensive), deals on housing, and other perqs. Salaries in India are closing in on the developed world, and go much farther than salaries in the US. Sure it might take a little while to start biting, but with things the way they are in the US right now, I’d bet more than a few companies would pull stakes and move.
India is not a export dependent economy to western countries. Even a recession in US will have the least effect on India vis-a-vis south-eastern countries such as China, Japan etc.
There is no strategic or regional gain to India by severing ties with Iran. India also needs all the oil it can import. Iran is not sending or supporting terror in India. All terror incidents in India have been traced to origins in Pakistan.