From a purely dollar stance, you would want the GC game to be against a Victorian club so it'll be beamed back live into Melbourne. If it's against Port or Freo then the tv networks won't want to show it and the national exposure angle for doing it is lost. Also you lose home ground/state advantage playing it against an interstate side. Playing St Kilda up there is a better option given at least Carrara is a neutral venue rather than the Dome.
I can understand the reasoning if the payoff for doing it is we receive 10 home games at the 'G on a significantly better stadia deal playing Coll, Ess, Carl and even Hawthorn twice and by 2011 when GC17 enters the comp we then get all 11 home games at the 'G. Otherwise it just looks a desperate grab for cash by a worried board at the expense of the footy team. These decisions can backfire if playing one less home game in Melbourne costs us a spot in the finals or better. I've seen it happen before.
There's only one sure way to boost finances - Winning games and playing Finals.