I have just returned from Goa, having spent 10 days in Agonda staying in a beach hut. We stayed with Madhu huts. The whole of the road through Agonda is full of beach hut accomodation, all much the same price. The ones right at the front are around 1,200-1,500 rupees per night, the ones not actually overlooking the sea are around 800-1,000 rupees per night. For anyone who hasnt had the experience of sleeping in a hut on the beach, it is a strange one! It goes without saying it is VERY basic and towards the end I was fed up of sand everywhere, missed a pool and no hot water. 10 days was probably too much but it was trial and error for us. Half that time would have been ample.
I have to say, tho, if you are looking for a very quiet break, with nothing to do, right on the beach and you don't mind not having hot water etc. it may well be the thing for you.
The owner and staff at Madhu were great and the restaurant was lovely. The food was some of the best we had in Goa.