numeric data type

inside the function i can not read the parameter or so it seems, this what is do:

float8 db_balance,in_chgval;

in_chgval = PG_GETARG_FLOAT8(2);

elog(INFO,"in_chgval = %0.2f",in_chgval);

The above elog, always shows cero, more over, i m also trying to read a
numeric from the database and substracted from the in_chgval parameter,
the operation always gives 0,, so apparently i can not read a numeric from a
data base:

db_balance = DatumGetFloat8(SPI_getbinval(SPI_tuptable->vals[0],SPI_tuptable->tupdesc,1,&is_null));

elog(INFO,"db_balance(%f) - in_chgval(%f) = %f",db_balance,in_chgval,(db_balance - in_chgval));

it shows all 0 ..... please any advise would be really appreciated. thank you!!