The Sikkim Government levies royalty even on trekking expeditions to the state. As of 2010, the trekking royalty follows:
The alpine trek royalty amounts to US$ 200.00 for a 5-10 member foreign team, US$ 300.00 for an 11-15 member team and US$ 400.00 for a 16-20 member team. The royalty for other trekking trips is 50% of the alpine trek royalty.
An individual Indian trekker pays INR 500.00 towards alpine trek royalty. The government levies royalty of INR 3,000.00 for a 5-10 member Indian team, INR 4,500.00 for an 11-15 member team and INR 6,000.00 for a 16-20 member team. Like foreign trekkers, the Indians also pay 50% less for other treks. However, a Sikkim-based group of trekkers pays only one third of the royalty levied on the other Indian expeditions. Students from Sikkim can trek free-of-charge.
The above trekking royalty applies to a trek of about ten (10) days. Besides the royalty, you have to pay entry fees to the national park / wildlife sanctuary and other fees. So budget accordingly.