Hi, here are some places you should visit in Sikkim
Yumthang Valley of Flowers
Khangchendzonga National Park
The cost would depend upon the places your are staying in and whether you are travelling in public transport or private taxi. If you are staying in decent hotels and using a mix of private taxi and public transport, it should cost you around rs.1500-2000/day. You can make your own travel plans as well for this location, but if you dont want the hassle of planning everything you can book with an organiser.