I'm off to Goa for a short break in a couple of weeks, so have been looking for a cheap airfare -- if such a thing exists these days!
Before booking, I always use www.makemytrip.com to compare the airfares offered by the different airlines. The best thing about this site is that it shows the total airfares, including taxes. This is particularly important in India because the airlines levy so many taxes, including a fuel surcharge, that add up to more than the airfares themselves. I usually book the actual ticket through the airline's website though, because it's often slightly cheaper that way without any service charges.
The good news is that many domestic airlines in India have recently reduced their airfares somewhat. Government operated Air India has decreased its fuel surcharge by 400 rupees ($8), while Jet Airways, Kingfisher Airlines, and Air India have removed transaction fees, which used to be levied at 300 rupees ($6) upwards.
However, I decided to fly with IndiGo, which is recognized as being the best low cost domestic airline in India. A one way ticket cost me 2,975 rupees ($60), including taxes. It wasn't much less than the cheapest ticket offered by a full service airline, which was 3,275 rupees ($66) with Air India, but any saving is worthwhile and the departure time suited me.