sed syntex in procmailrc question

I have a recipe wish 'sed' in one of my Smartlist rc, like:

    sed 's/SUBJET/'"$SUBJ"/g

which works fine.  I forgot where did I copied it from but have been 
puzzled by its syntex with respect to the use of the single quotes "'". 
Following what's described in sed tutorials I made it:

    sed 's/SUBJET/"$SUBJ"/g'

and it fails.  Could someone kindly point me to a "right" resource I 
should check? (I wondered in between sh/csh shell scripting syntex 
and general RE, among several other resources having things "alike"; Or 
is it really a 'sed/awk' thing that I have not really get into?)

Thanks in advance,

Zhiliang