If you are travelling solo to sikkim Then from jalpaiguri to gangtok.. you can book your single seat in wizzride for 750 INR per seat. Hotels are as cheap as 450 INR per night. Not bad for a single person budget trip.
Travelling to North sikkim becomes costly if you don't hv group. Cover east sikkim along with gangtok and have a nice trip. Upvote if this helped.