Bhutan Telecom offers both prepaid and postpaid options for its DSL service. Let's start with the prepaid plans. There are 5 plans, starting at Nu 399 for 2.5GB (the "Personal" package) and going all the way up to Nu 2499 for 15GB (the "Enterprise" package). Much like mobile talk plans, the data quota has a validity of 30 days, but if you renew before expiration - any leftover data quota is carried over.
When it comes to price per GB, there is no advantage to choosing the more expensive plans - they all charge between Nu 160 and Nu 180 per GB (with the 10GB plan being the most expensive). However, the packages differ in their maximum theoretical speeds. The cheapest ones offer 256kb/s, while the most expensive plan offers a theoretical speed of 2Mb/s, or 2048kb/s, which is 8 times speedier than the cheapest package. But again, this is in theory. When the international network is congested, you are out of luck even if you bought the most expensive package.
If you only use the Internet for occasional email, chatting, and browsing, the cheapest package - Nu 399/2.5GB/30 days - will probably suffice. Even if you run out of data, you can always recharge. In fact, even if you download music, I still recommend that you start with the cheapest package. 1GB is equivalent to around 300 typical MP3 songs. If you find that you keep running to the BT One Stop Shop for recharging every week or two, consider upgrading to a more expensive package.
What about the postpaid plans? Again, there are five plans, and they are equivalent in quota and speed to the prepaid ones. They are more expensive than postpaid - between Nu. 190 and 216 per GB, but they do offer a certain convenience - you will never run out of data (you pay Nu 0.19 per extra MB after your quota is finished, which is Nu 195 per GB). I am not in favor of the postpaid plans, especially since the bills keep piling up when you leave town for a few weeks and do not use the service.
Last but not least: You can always check your DSL balance and expiration date here.