Why is Starbucks so expensive?
Three parts to this question:
Why is the cost structure of Starbucks high;
Why are they able to charge such high prices;
And why do they choose to charge such high prices?
Regarding the cost structure, as others have pointed out, they have a high cost model for producing coffee beverages (and other products).
Principally, they are often located in high rent areas, and are optimized for service quality rather than cost-efficiency.
Regarding how they are able to charge such high prices, it's largely a product of branding and convenience.
Though I wouldn't say they are out of line with other independent coffee shops, so the branding perhaps isn't even that marginally important, other than attracting customer volume.
Regarding why they choose to charge such high prices ... because they can.