It will entirely depend on where you want to stay in Goa and for how many days. In peak tourist season the hotel rates will be high, but still for a standard itinerary of 4 days, 15000 should be sufficient. I would recommend first decide which places you would want to visit in Goa and book hotel nearby. you have option of renting a bike which will not cost you much.
Depends on what you are looking for if you want only travel to beaches and no party then only food and hotel expense you need. If you have advance hotel booking in cheap price then it will.be around 4k trip for 2..3 days with good budgeted hotel and food. If you are looking for parties then accordingly it will increase.
For 4 days, minimum 20k(including flight, stay, food, water sports, booze) you need. This is for on season. you can save around 5k if you travel by bus. In Off season obviously there won't be any watersports, beach side shacks, and most of the beach side restaurant will be closed. In off-season within 10 to 15 k you can travel to Goa depending on whether you travel by bus or flight ( flight charges in off-season is very less almost equal to bus fare).