Hi, Apeksha!
Budget for a week-long Goa trip depends a lot on how you wish to travel, your accommodation choices, and what all you wish to experience. However, anything between ₹14,000-₹17,000 (excluding flights) should be enough for a holiday in Goa that is supposed to last around a week.
Hope that helps.
Don't visit in December. Too much crowd due to the eve of New Year. Visit in January.
Still if you want to visit in December, the budget should be around 15-20k.