You could also buy butter, sugar, flour, eggs, etc., and make your cake from scratch. Good cookery books tell you how much batter will fit in what size of tin.
Once you've bought the basic ingredients you'll have enough for more than one cake so it's cheaper, as well as tasting far and away better and being better for you, than anything in a packet that includes all sorts of chemicals, stabilisers and what-not.
Don’t look for the light at the end of the tunnel. Stomp along and switch the bl**dy thing on yourself.