I heard that access to Bhutan is quite restricted for all people of outside South-Asian countries? Can I have the details on that?
This is indeed true. What HaLaBi's answer misses out on - and this is what makes it expensive rather than difficult per se - is that the Bhutan government sets a minimum daily tariff that tourists are supposed to spend when booking with a travel agent (and visas can only be obtained once a tour package has been booked with a travel agent).
The details are listed on the Bhutan tourism website.
The minimum tariff for tourist visiting in a group of 3 persons or more are as follows:
High Season Tariff – USD$ 250 per person per night for the months of March, April, May, September, October, and November.
Low Season Tariff – USD$200 per person per night for the months of January, February, June, July, August, and December.
The minimum price includes:
All internal taxes and charges (including the royalty) Accommodation All Meals All travel with a licensed Bhutanese Tour Guide All Internal Transport Camping Equipment and Haulage for Trekking Tours
The rates given above are applicable per tourist per night halt in Bhutan. On the day of departure, the ‘local agents’ host obligation shall be limited to breakfast only and any extra requirements shall be payable on actual basis.
Couples or solo travellers have it slightly harder:
Individual tourists and smaller groups of less than three persons shall be subject to surcharge, o*ver and above the minimum daily rates applicable*, as follows:
Single individual US$ 40 per night Group of 2 persons US$ 30 per person per night
The 10% agency commission payable to agents abroad shall not be deductible from the surcharge
So while the visa is not harder to get, Bhutan controls the influx of tourists using this pricing strategy. It's part of a bigger policy the Bhutan government has on regulating its policies to measure 'Gross National Happiness' (GNH) rather than GDP.
Obtaining Bhutanese visa for citizens of other than India, Bangladesh and Maldives seems quite difficult. According to the official website of Tourism Council of Bhutan you will need to book a tour with a local licensed tour operator directly or through a foreign travel agent which will submit the visa request for you:
Other than Indian, Bangladeshis and Maldivian nationals, all visitors to Bhutan require a visa; all visas are issued from Thimphu; visas are only issued to tourists booked with a local licensed tour operator, directly or through a foreign travel agent. Applications for tourist visas are submitted by the tour operator. Visa clearance from Thimphu must be obtained before coming to Bhutan. Visa clearance takes at least 10 days to process. Air tickets to Bhutan cannot be purchased without visa clearance. At your point of entry the visa will be stamped in your passport on payment of US$20, two passport photos will also be required. Visas are issued for a 15day period; extensions can be obtained in Thimphu at a cost of Nu.510.