In the first <xsl:when>, you need a slash (/) between "." and
"DisplayPrice".  Because those two items together without a slash
between are not a valid XPath "sub-expression", which is what the
parser would expect to find between the parenthesis of a function
call, the parser assumes that the sub-expression must end after the
".", so it "expects" the closing ")".

-Brandon :)

An XML name cannot begin with ".". So after

number(.

the only things that can legally come next are either ")", or an
operator such as "+", "div", or "/".

It's not a good message. Sometimes when you get a poor error message,
it's worth trying another XSLT processor to see if it gives you a better
one. Saxon on this one gives you:

Error at xsl:when on line 24 column 67 of test.xsl:
  XPST0003 XPath syntax error at char 9 on line 24 in
{(number(.DisplayPrice)}:
    expected ")", found name "DisplayPrice"

The "expected ')'" when there are many other symbols that could occur is
symptomatic of a top-down parser, which typically only reports what it
was expecting as a 'last resort'. A bottom-up parser that builds a
finite-state machine would know all the possible symbols that could
validly occur in this context.

Michael Kay
Saxonica