For the time being, they are targeting specialty applications where it is not uncommon to charge in excess of $1000 per meter even for normal earthly fibers.
For scaling up, if one uses the smallest commercial fiber standard (80um diameter cladding--they would put the buffer and whatnot on after it comes back to earth) the mass of the fiber is only 53grams/km so using your $3000/kg the cargo cost is only $0.16/meter which seems pretty palatable to me.
That's kind a obvious: high-frequency trading. Thousands of banks and companies will happily to pay premium to get some edge over competitors. As soon as single entity on market have it everyone else would want it as well so there will be plenty of demand.
For scaling up, if one uses the smallest commercial fiber standard (80um diameter cladding--they would put the buffer and whatnot on after it comes back to earth) the mass of the fiber is only 53grams/km so using your $3000/kg the cargo cost is only $0.16/meter which seems pretty palatable to me.