Crystal Reports formating issue

Message

     Hi,

In my report, I wish to conditionally reformat the values based on different invoice types: Main, Debit, Crredit. 
I have a formula where I do some calculations to get value to display in the report, e.g.

Item     INV TYPE        Actual INV COST

A           M                      $140
A          DB                      $20      

B          M                       $45
B          CR                      $5

How can I formt the value with DB and CR to ($20), ($5)  --surround with a (). This Actual Inv Cost was obtained throught a formula:

if {CCY.ANSI} <> 'SGD' then
  {@get_foreign_$} * {CCS.BNKRATE}
else
  {PRC.UNIT} - {PRC.DISCAMT} + {PCH.AMT}

NOTE: the above lines doesn't take into consideration of quantity, that's fine for this circusmstances.

I have searched around and can find anything useful in the "Format Field"
Could anyone please advise about the above, thanks in advance.

John