it is all about electromagnetic fields in your systems which depends on kinf of nano particles and morphology you have used.

You should do some simulations to see your electromagnetic behavior of excited and scattered light in your system and then calculate the ration of both which  means enhancement factor.

In a simple way you can estimate the analytical enhancement factor=(I SERS/I RAMAN)*(C RAMAN/C SERS)

where I SERS is the area under the Raman peak in SERS spectra for a particular mode at a concentration C sers;I RAMAN is the area under the Raman peak in spontaneous Raman spectra of the same mode at higher  concentration C RAMAN;

Another way is there to estimate ennhancement factor via measuring adsorption factor, no of molecules those attributed to SERS and Raman spectra