Postgresql can not understand alias column

In this pgexercises about joining 3 different tables, the answer is given as following:

select mems.firstname || ' ' || mems.surname as member, 
    facs.name as facility, 
    case 
        when mems.memid = 0 then
            bks.slots*facs.guestcost
        else
            bks.slots*facs.membercost
    end as cost
        from
                cd.members mems                
                inner join cd.bookings bks
                        on mems.memid = bks.memid
                inner join cd.facilities facs
                        on bks.facid = facs.facid
        where
        bks.starttime >= '2012-09-14' and 
        bks.starttime < '2012-09-15' and (
            (mems.memid = 0 and bks.slots*facs.guestcost > 30) or
            (mems.memid != 0 and bks.slots*facs.membercost > 30)
        )
order by cost desc;

What I don't understand is that why can't I refer to the SELECT cost alias at the WHERE clause? If i run the same query with

        where
        bks.starttime >= '2012-09-14' and 
        bks.starttime < '2012-09-15' and
        cost > 30
order by cost desc;

then an error occurs:

ERROR: column "cost" does not exist

It's clear with me from this answer that it's because of the order of evaluation. But why order by cost desc; is allowed?