I had already booked my flight but my friend had not , so when he went to book a flight from Bombay to Goa it was twice the price of mine and the only seat left . But that was with Spice jet and you book at the main front entrance to the domestic airport . But if you have not got a domestic ticket you cant use the free shuttle bus from the international to domestic .
I booked my domestic flight with http://www.cleartrip.com/ No difference in price if i booked it with Indian Air
Asda travel are about £8 more expensive for the same flight and times as i have .
Oh and Indian Air have got a guarantee for the Indian government to pay for the fuel they are using .

Have never booked flights at mumbai but find that asda travel site on the web is very good,booked our flight to goa on this,very good price and very quick service! cheers jantie