In short:

$specialprice="Null";
$query = "UPDATE mytable SET mytable.specialprice = '".$specialprice."'
WHERE ......";
submitted a "0"(zero) into mysql.

and:
$specialprice="mytable.specialprice =Null";
$query = "UPDATE mytable SET '".$specialprice."'
WHERE ......";
submitted Null, like it should.
I couldnt account why the first version would submit "0"

the mysql field has the following properties:
type:float,
length=11,
decimals=2,
allow null:true,
unsigned: false,
zerofill: false,
default value:empty