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.
Perhaps the internet price excludes the EW payload? Seems like a plane with a load of electronics gear and transmitters/antennas would cost more than the same plane sans that stuff?
I do not know the specifics, but I think most of the EW gear sits on pods attached to the pilons, not inside the actual plane. The only difference that I know of compared to a regular F18 is that it requires two crew members to operate.
Most of the Growler "magic" is in pods carried on external hardpoints, but you couldn't just upload the EW pods onto a FA-18F Super Hornet and have a Growler.
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).
