Oo, interesting question! I think there must obviously be an international trade in magical objects, materials, ingredients etc. There must be small enterprises, making the honeyduke's sweets, making parchment, robes etc. The WW has shrunk so much, at least in Britain that any large-scale operations or land-holdings would be too difficult to hide. I imagine there must always have been tie ins to the muggle economy, land and property ownership and investments. It is perfectly possible to keep the statute of secrecy and maintain a substantial portfolio of muggle assets. I'm sure the goblins would be adept at maintaining an untraceable fog of off-shore accounts, trusts and companies obscurely registered. Who knows, maybe the whole off-shore tax haven system was initiated by wizards :D