This is basically a customer to customer forum. Cost will usually be determined on market area. Your best bet will be to call comcast billing dept or speak to the retention dept. they might be able to give you a deal.
if you have HD now and go to the adapters which are non hd you will not be happy as the picture is not very good at all. If you do want to go that route you will be changing your package also as you won't need hd anymore so you don't need an hd package. you probably pay around 15.00 each for each box if its a dvr, and you have to pay the 10.00 hd technology fee. If your in a promo you cannot do this as your locked in and would incur a penalty. LIke reasd said only your local office can tell you what your new bill would be but I would say you will be able to knock it down more than 50.00 if you don't mind the picture.