Please read, “How shakuhachi are priced and valued” for specific information on shakuhachi pricing.
It is best to acquire a shakuhachi through a credible teacher. In any event it is wise to only buy shakuhachi from makers or sellers who actually understand what an authentic shakuhachi is. This is because of the fact that, unlike many instrument, you cannot usually see the flaws of a shakuhachi. A “shakuhachi” may have five finger-holes and a root-end but these are little indication if even the most basic notes will work.
To put it into perspective, an incorrectly made “shakuhachi” would be like playing half of a piano or less. In any case, a reasonable auditioning period should be offered in which someone with more experience, such as a shakuhachi teacher, can test the instrument(s) out for you and offer more insight.