Five million Aussies will celebrate Halloween today and spend an average of $86 each getting spooky, according to the first survey of its kind by the Australian Retailers Association (ARA). That will give retailers a $430 million boost, with people aged 35-49yo the most likely to join in with the festivities. And it’s not just parents: 20% of those people will host or attend a party without kids. ARA boss Paul Zahra says Halloween has grown in popularity in Oz, and many shop owners are joining in with themed windows and store displays. But there’s no doubt the US remains the epicentre of All Hallows’ Eve, with Americans spending a whopping $16.5 billion on the annual celebration of all things macabre, including $1.1 billion on pet costumes alone, even though the evidence suggests they hate it…