North Sikkim packages depend on the time you travel because of the weather conditions. The packages are of 2 nights 3 days and 1 night 2 days. If you travel in a sharing vehicle it will cost around 3000-3500 per head and if you take a package including vehicle, food and stay it can be around 25k-30k. But again it will depend on the number of persons. Better to contact I travel agency before reaching Gangtok because you will be needing travel passes to visit North Sikkim and generally it takes one day to make the pass. If you don't contact any agency then take one day extra in hand to make the pass. There are a number of travel agencies in Gangtok you can contact them when you reach there.