$200-$300 from Delhi ?
Travelocity.co.in as you can book a LCC...
Have you tried booking to BOM? It's always cheaper from the UK... I am currently in Cochin (COK) and I am travelling to Goa tomorrow as its cheaper to fly to COK than GOI... GOI is a small airport.
If cost is an issue, get the train or bus from BOM as it's only over night..
BTW don't go to North Goa.... South Goa is the place to be!
Cundall..
PS Sunburn festival will be in North Goa around this time so you need to book the flights if you do fly ASAP.