Yeah, I thought it might be like the cost of an aircraft carrier multiplied by the size of the star destroyer, but they’ve just got the in-universe price as quoted in one of the EU books I guess?
For the record, assuming the star destroyer is 5 times as big as the Nimitz in every dimension, I’d make it about $1 trillion at the same cost per pound to make one that doesn’t fly.
To scale up the ISS to the size of a star destroyer, you’re talking maybe $4.5 quadrillion all up, but that’s a space station, not a space ship.
A better comparison might be the space shuttle. If we assume a star destroyer is 6250 times the mass of the shuttle, then it’s only $1.25 quadrillion.