The data type is Currency or Date/Time can be set the formatting. Available in Ver.2.0 or later.
Word/PowerPoint/PDF
e.g. {!Case.TestCurrency__c, $#,##0.0, HALF_UP}
Value: 1234.45
Result: $1,234.5
Excel
Merge fields cannot be formatted. Set from Excel's cell formatting.
Instruction manual
Formatting of Currency and Number Fields