If you look at the F-18E, it is not so cost-effective after all. It costs much more than the C/D. However the reduction of RCS, combined with the increased range and the increased bring-back capability make the program acceptable.
For the F-16, there is no problem of range - with CFTs in particular -, thrust, and bring-back capability. Redesigning the aircraft completly to only reduce the RCS - that's what it would take - would probably not be cost effective.
However, it might be feasable to reduce its RCS a bit more by using more RAM coating and more composite material I don't really know. This might be cost effective against non stealthy aircraft if it increases significantly the chances to have the first shot.