That price depends on how long a piece of spring you are purchasing. I buy the stuff in 50 foot rolls for my upholstery business. $30.20 is a very good price if that is a 50 ft roll of 9 gauge, as thats the wholesale price from my supplier.
As for the foam, what is used in production vehicles is a very dense foam, and few places outside of auto upholstery suppliers handle it. As for the foam, you will want whats called Super and that weighs 2.5 lbs per cubic foot. At least thats the densist foam I can get.