The \b and \f switches don't work with other switches. Instead, you could code the mergefield as:
{ MERGEFIELD Cost \# "'$→'0.00' '; '$→'(0.00)"}
or:
{ MERGEFIELD Cost \# "'$→'0.00; '$→'-0.00"}
and use a right-aligned tab in the cell.

To suppress 0 outputs, you could code the mergefield as:
{ MERGEFIELD Cost \# "'$→'0.00' '; '$→'(0.00);"}
or:
{ MERGEFIELD Cost \# "'$→'0.00; '$→'-0.00;"}

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]