Actually, this is better accomplished with class instance variables:

class Subscription
    def price_with_discount
        class << self.class
            @price
        end * 0.9
    end
end

class BasicSubscription < Subscription
    class << self
        @price = 19
    end
end

class PlusSubscription < Subscription
    class << self
        @price = 49
    end
end

p BasicSubscription.new.price_with_discount
p PlusSubscription.new.price_with_discount

Using @@class variables will change the price of all subscriptions when 
it's set, so
all subscriptions would end up being $49.

You can also change the @price above to PRICE to use class instance 
constants,
which is probably what you want.

- Dan