If the Internet is to be believed they're not actually more expensive than an F/A-18, and as far as military aircraft go.. not the most expensive. But a ~$150M accident is nothing to sneeze at.
The cannon in the nose of a regular Hornet is replaced with computer hardware in the Growler. There are also aerodynamic changes to make it a little more stable so it's a better EW platform. There's probably a million other small differences too, enough that you wouldn't try to convert a -19F Super Hornet into a Growler (although the RAAF did think they'd try at one point).