PARAM="$SITE,90,1000"
PARAM=${PARAM//'$SITE'/$SITE}

PARAMS="${PARAMS//"$SITE"/$SITE}": 0403-011 The specified substitution is not valid for this command.

PARAM=$SITE${PARAM#$SITE}

> SITE=12
> PARAM='$SITE,90,1000'
> echo $PARAM
$SITE,90,1000
> echo "$SITE${PARAM#$SITE}"
12,90,1000

$ SITE=42
$ PARAM='$SITE,90,1000'
$ echo ${PARAM//$SITE/$SITE}
42,90,1000
$ echo ${PARAM//'$SITE'/$SITE}
42,90,1000
$ ksh --version
  version         sh (AT&T Research) 93u 2011-02-08