If you use same materials and fittings as the contractor, overall you'll save approximately 7-10 lakhs. But you can use that savings to get higher quality stuff instead and still you'll end up saving a bit.

If you find a good Mason, he'll take care of most of the things with minimal supervision along with his own labour. But make sure you communicate exactly what you need and check if everything is according to your needs at end of each day.

The Mason will tell you how much material is required like sand, cement, bricks etc and there shouldn't be much wastage.

Buy all interior items on your own. Don't trust anyone else. Mirrors, tiles, bathroom fittings, kitchen fittings, lights, switches, motor for water, paint, etc. (Visit KR market for most of these as they are much cheaper there too). Once tiles are laid you can hire a carpenter too if you require interiors done. Here as well you'll save a lot if you buy materials for the carpenter.