XSL problem using sum() function

Im trying to get the sum of all salaries for company:BG (see XML
below).  Im using translate to remove the '$' and ',' from the dollar
amounts and then, trying to sum them up.  But i get the error that it
cannot convert String to NodeList.  I've tried some other workarounds
but i just cant seem to get it.   Any ideas?
Thanks
Chris


The XSL:

<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0">

        <xsl:output method="html"/>

        <xsl:variable name="workssection" select="document('works.xml')" />
        <xsl:variable name="empsection" select="document('employees.xml')" />

        <xsl:template match="/">
        <xsl:for-each select="$workssection//works-for/" >
          <xsl:if test ="company-name = 'BG'" >
            <xsl:variable name="trans" select="translate(salary,'$,','')" />
            <p><xsl:value-of select="sum($trans)" /></p>
          </xsl:if>
        </xsl:for-each>

        </xsl:template>
</xsl:stylesheet>



A portion of the XML file:
<works>
      <works-for>
            <emp-name>John Doe</emp-name>
            <company-name>BG</company-name>
            <company-city>Ames</company-city>
            <salary>$70,000</salary>
      </works-for>

      <works-for>
            <emp-name>Margaret Thatcher</emp-name>
            <company-name>BG</company-name>
            <company-city>London</company-city>
            <salary>$7,000,000</salary>
      </works-for>
      ....