How can an Indian startup use Authorize.net with a U.S. bank and merchant account?

Very few processors first of all work internationally, and even fewer will work on variably billing. Even if they did decide to work on variable billing, it would need to be capped with some limit.

I know Authorize.net will not work directly with you, since you are off States side. You might want to search for companies who are 3rd party processors and explain your model to them. They just might work with you.

Another alternative would be to look at Plimus - www.plimus.com - always best to ask.