In their morning fund raising drive, Colorado Public Radio stated that they have 350,000 listeners and about 30,000 members (who make some sort of donation). Thus, about 10% of the adult population listens.
The widespread assumption which I am sure some data supports, is that the audience is very elite in education, socio-economic status, income and wealth -- even the BBC's reporting style tends to target its broadcasts towards a more general audience and is less relentlessly serious. Most of the content on Colorado Public Radio is purchased from other providers like National Public Radio and Public Radio International, although there is something like an hour of local news content each day, and in addition on the second channel which plays classical music, some DJ chat.
It receives a small percentage of its support from government grants, and makes some money from corporate underwriting (i.e. discrete advertising), but earns a substantial share of its revenues from listener donations.