Not using keys:

<xsl:template match="cdlist">
<xsl:for-each select="cd">
<tr>
<td><xsl:value-of select="artist"/></td>
<td><xsl:value-of select="title"/></td>
<td><xsl:value-of select="@price"/></td>
<xsl:variable name="pricecolour" select="@price" />
<td>
<xsl:for-each select="/cdlist/pricelist/price">
  <xsl:if test="$pricecolour = @ticket">
    <xsl:value-of select="." />
  </xsl:if>
</xsl:for-each>
</td>
</tr>
</xsl:for-each>
</xsl:template> 
     or
<td><xsl:value-of select="/cdlist/pricelist/price
[@ticket = $pricecolour]"/></td>
         

Each entry has to look up the complete set of possible prices to find the correct one

Result