Hey Alex- Great you are planning a trip.
All travel to Bhutan has to be booked through an accredited Bhutan travel operator. There is no independant travel. They manage the visa and the spend.
The government has a minimum spend of $200 USD per day in low season and $250 USD per day in high season. That includes accommodation, all meals, personal tour guide and personal driver. Plus admission to the main sites.
By enforcing eveyrone to go through one of their licenced operators they control the numbers, the visas and the spend.
Shout out if you need more info.