If you are going for one week in peak season then it will be more costlier. Otherwise you can stay in huts or hotel or OYO room in cheap rates. under 800rs per day. Hiring a scuty /bike will cost you 300rs per day. Rest depend on your comfort zone about clubbing and drinking and fooding where you want to eat.
Depends on num of days but you can calculate based on below: During off season Avg stay per night : 1000, Avg food per day : 400-600, Travel within Goa via vehicle (Activa) per day: 250 + petrol. And during peak season increase in Average cost by 25%.