Formatting can be set for merge fields whose data type is Currency or Number. Available in Ver. 2.0 or later.
e.g. {!Case.TestCurrency__c, $#,##0.0, HALF_UP}
Value: 1234.45
Result: $1,234.5
*In Excel, merge fields cannot be formatted. Set the format from "Format Cells".
Instruction manual
Formatting of Currency and Number Fields