I copied code from Breck's blog, and did a small change to accept 3 decimals.

ALTER FUNCTION "DBA"."dollar_number_string" ( IN amount DECIMAL ( 30, 3 ) )
RETURNS VARCHAR ( 50 )
BEGIN

DECLARE string_amount    VARCHAR ( 500 );
    DECLARE insertion_point  INTEGER;

DECLARE @sqlcode         INTEGER;
    DECLARE @sqlstate        VARCHAR ( 5 );
    DECLARE @errormsg        VARCHAR ( 32767 );

IF amount IS NULL THEN
       RETURN NULL
    END IF;

SET string_amount = STRING ( 
       IF amount BETWEEN -.99 AND -.01
       OR amount BETWEEN  .01 AND  .99
          THEN '0' 
          ELSE '' 
       ENDIF, 
       ABS ( amount ) );
    SET insertion_point = LENGTH ( string_amount ) - 6;

WHILE insertion_point > 0 LOOP
       SET string_amount = INSERTSTR ( insertion_point, string_amount, ',' );
       SET insertion_point = insertion_point - 3;
    END LOOP;

RETURN STRING ( 
       IF amount < 0.00 
          THEN '-'  
          ELSE ''  
       ENDIF,
       '',
       string_amount );

EXCEPTION
       WHEN OTHERS THEN
          SELECT SQLCODE, SQLSTATE, ERRORMSG() 
            INTO @sqlcode, @sqlstate, @errormsg;
          MESSAGE STRING ( 
             'EXCEPTION in dollar_number_string at ', 
             CURRENT TIMESTAMP, 
             ': SQLCODE = ', @sqlcode,  
             ', SQLSTATE = ', @sqlstate,  
             ', ERRORMSG() = ', @errormsg ) 
             TO CONSOLE DEBUG ONLY;
          RETURN '???';
END

ALTER FUNCTION "dba"."dollars"( in a decimal(30,3) )
RETURNS varchar(50)
BEGIN
    declare s varchar(50);
    declare i integer;

If a is null then
        return null
    end if;

set s=string( if a between -.99 and -.01 OR a between .01 and .00 then '0' else '' endif, abs(a));
    set i=length(s) - 6;
    while i > 0 loop
        set s=insertstr(i,s,',');
        set i= i - 3;
    end loop;
    return string( if a <0.00 then '-' else '' endif, '', s );
END

And here is the code to test both :-

   SELECT *
     FROM (
   SELECT CAST ( NULL AS VARCHAR ( 100 ) ) AS "input", dollar_number_string ( NULL )    AS "output", dollars( NULL ) AS "output2"
   UNION ALL
   SELECT '-0', dollar_number_string (  -0 ), dollars (  -0 )
   UNION ALL
   SELECT '-.1', dollar_number_string (  -.1 ), dollars(  -.1 )
   UNION ALL
   SELECT '-.99', dollar_number_string (  -.99 ), dollars(  -.99 )
   UNION ALL
   SELECT '-.994', dollar_number_string (  -.994 ), dollars(  -.994 )
   UNION ALL
   SELECT '-.995', dollar_number_string (  -.995 ), dollars(  -.995 )
   UNION ALL
   SELECT '-1', dollar_number_string (  -1), dollars(  -1)
   UNION ALL
   SELECT '-1.1', dollar_number_string (  -1.1 ), dollars(  -1.1 )
   UNION ALL
   SELECT '-23.45', dollar_number_string (  -23.45 ), dollars(  -23.45 )
   UNION ALL
   SELECT '-123.45', dollar_number_string ( -123.45 ), dollars( -123.45 )
   UNION ALL
   SELECT '-9123.45', dollar_number_string ( -9123.45 ), dollars( -9123.45 )
   UNION ALL
   SELECT '-99123.45', dollar_number_string ( -99123.45 ), dollars( -99123.45 )
   UNION ALL
   SELECT '-999123.45', dollar_number_string ( -999123.45 ), dollars( -999123.45 )
   UNION ALL
   SELECT '-9999123.45', dollar_number_string ( -9999123.45 ), dollars ( -9999123.45 )
   UNION ALL
   SELECT '-123456789012345678.90',  dollar_number_string ( -123456789012345678.90 ),  dollars( -123456789012345678.90 )
   UNION ALL
   SELECT '0', dollar_number_string (  0 ), dollars(  0 )
   UNION ALL
   SELECT '.1', dollar_number_string (  .1 ), dollars(  .1 )
   UNION ALL
   SELECT '.99', dollar_number_string (  .99 ), dollars(  .99 )
   UNION ALL
   SELECT '.994', dollar_number_string (  .994 ), dollars(  .994 )
   UNION ALL
   SELECT '.995', dollar_number_string (  .995 ), dollars(  .995 )
   UNION ALL
   SELECT '1.00', dollar_number_string (  1.00 ), dollars(  1.00 )
   UNION ALL
   SELECT '1.1', dollar_number_string (  1.1 ), dollars(  1.1 )
   UNION ALL
   SELECT '23.45', dollar_number_string (  23.45 ), dollars(  23.45 )
   UNION ALL
   SELECT '123.45', dollar_number_string ( 123.45 ), dollars( 123.45 )
   UNION ALL
   SELECT '9123.45', dollar_number_string ( 9123.45 ), dollars( 9123.45 )
   UNION ALL
   SELECT '99123.45', dollar_number_string ( 99123.45 ), dollars( 99123.45 )
   UNION ALL
   SELECT '999123.45', dollar_number_string ( 999123.45 ), dollars( 999123.45 )
   UNION ALL
   SELECT '9999123.45', dollar_number_string ( 9999123.45 ), dollars( 9999123.45 )
   UNION ALL
   SELECT '123456789012345678.90',   dollar_number_string ( 123456789012345678.90 ),   dollars( 123456789012345678.90 )
   ) AS testing
   ORDER BY CAST ( "input" as DOUBLE );

Here is the output :-