It depends on your way to travel. If you need a comfortable trip 25k - 30k for North and East sikkim including private taxi. If you want a budget trip use public transport from new jalpaiguri or siliguri railway station, and reach Gangtok, for North sikkim you can book a trip with a local travel company.
that can be around 12-20 k depends on your budget.
Hey! I had the same question before planning my NJP–Darjeeling–Sikkim trip. I came across a story on TravelStoriz.com where someone had shared their full itinerary with prices and local tips — that really helped me figure things out. Based on that, I booked a 5-day package for around ₹9,000 including shared cab from NJP, budget stays in Darjeeling and Gangtok, and basic sightseeing. I followed a similar route and it worked out really well for me. I personally got answers like this from TravelStoriz.com, so definitely check it out — it made my planning way easier and more real.