The main reason Y class exist is not to give you better odds for upgrades...
There is 99% of the times when you will always find a seat in Y than in S or any lower class (if you go economy).

Also if you are a PL and there is a fully booked flight in economy, And you
can no longer order ticket this flight, You can alway book a Y class ticket,
and ELAL will manage who they will take off from the flight, This is really
useful for me, I have used it more than one time and is a life saver...

So being upgraded thru Y is only a bonus... 8-)
As for prices, for Corporates the same discount apply both to S and Y,
And to those of you that really think corporates get an amazing discount,
that's not true, The discount is so low and only on high classes (S,Y, I,C,F).