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.
Couples or solo travellers have it slightly harder:
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: