Lovely as it is, Sapa is a bit more expensive than Hanoi or most of the rest of Vietnam. Basic hotel rooms can be quite cheap, but once you want even a little luxury then the price starts jumping up quickly.
Food and drink are a little pricier than in Hanoi, and many of the restaurants lining the main tourist street will seem quite expensive if you are used to things in the capital.
Another thing that can quickly throw your budget off is the tours of nearby attractions and craft villages. Unlike most other places, there seem to be few organized tours to join, so it’s often a matter of hiring a guide/driver/vehicle yourself for half a day or a full day. If you are in a group of 3 or more people then it can work out to a decent deal, but as a solo or a pair it can seem fairly expensive.