Since Comcast encrypts a lot of channels all tvs in a house will need a box to receive channels above LImited Basic tier. A DTA would work also, these were made for older tvs without digital tuners.
Can't help on the cost since costs vary by each Comcast market and this is mainly a customer to customer help forum. Even the Comcast employees that help here may just have your local office call you.
For pricing the safest bet for installation costs would be to call your local office.
