I doubt you would see additional cost for an iPod/iPhone - more likely the existing price structure will stay static for longer than it would without the subscription model (i.e. less price drops for iPhone/iPod products) so introducing extra cost in a hidden sort of way.

Personally I don't like the idea of a subscription model for music. Surprised if this does go ahead as iTunes movie rentals do not follow a subscription model, rather one-off rental costs.