Flights to Paro in Bhutan are notoriously expensive. There is only one airline that flies in and out: Druk Air. They have a complete monopoly so there is no incentive for them to reduce their fares. They openly state that if the air fare increases after the date when you bought your ticket they will collect the difference from you when you check in. There are no discounts except for citizens of Bhutan itself.
If you are thinking of avoiding the expensive air fare costs by crossing into Bhutan by land then you need to think again. The Department of Tourism (DOT) state that visitors must either enter or leave the country via a Druk Air flight. Only Indian national are exempt from this rule. The only land border crossing from India into Bhutan that allows travelers to cross both ways (enter and exit the country) is at Phuentsholing, which is on the border with West Bengal state of India. Visitors can also use Samdrup Jongkhar to enter Bhutan, but not to depart the country. Ensure that you get your passport stamped when leaving India.
There are quite a few Bhutanese travel companies which offer direct bus services between Siliguri and Phuentsholing. Most of them depart twice a day.