First, let me say you are starting out right by replacing the galvanized supply pipe. There are a couple of choices for this pipe, but I would let the plumber decide what he prefers. It might be PVC or copper, but it would not be CPVC. Now for the bad news. You really should consider replacing all of the galvanized pipe. There's not exact life span for galvanized pipe, it does depend on the water quality, but 40 years is a ballpark. Inside the house, you can not use PVC for water supply (drains yes) CPVC would be legal, but it would need to be sized larger that the galvanized it is replacing as the inside is smaller. Copper would be a good choice, and many are using PEX. The question about price is like me asking you on line how much it would cost to fix my car, or paint my house. Not seeing the job and what all it would require makes this an impossible question to answer on line. Not only that, local labor prices vary widely from place to place. Having dug the supply line trench will save you some money, but that's only part of the job. It's always wise to get at least 3 estimates from local contractors who can see and measure the job site. Also, check around with friend and neighbors who may have used a local company and see what they can tell you about their work. Just be sure you deal with a licensed company and not a handyman that is a jack of all trades and works for cheap. Cheap is nice, but it usually comes at a price.