Scorpii/Bolero etc. will charge Rs.4000 per day. But I would suggest for hiring car only for transfers (as and when required). Nathula/Baba Mandir/Tsongo Lake and North Sikkimto be done in seperate package which includes vehicle also. They wont allow any vehicle other than they provide. Taking a single vehicle for the entire trip is not worth in Sikkim.
Sikkim has different rules and regulations regarding transportation. You have to take local no taxi for local sightseeing. It will cost around 2500 for 4 seater and 3500 for scorpio or Innova per day. There is other options also you can go sightseeing in shared taxi which will cost around 500 to 600 per day.
And it's better to take taxi cause the roads are now more dangerous due to monsoons. And roads on upper part of Sikkim is not safe for drivers who drive on plains.