I am looking to buy a home for a great price - however it needs piping - if I can low-ball the purchase price -I am willing to use the difference to put in the piping and cosmetic work
How many square feet is your house? How many faucets, toilets,showers, dishwasher,washer, etc. Until you know how much material you will need besides the pipe the amount for materials can vary greatly. Labor is what is going to eat you alive, twice the amount of materials. Why don't you consider using Pex, as it cheaper than copper, has a life time guarantee and you can run a single line to every tap. It's very hard to say without knowing the size of the house, how many bathrooms etc. I would try to get the seller to come down between five to eight thousand less to have the work done.
It is rare that copper piping needs to be replaced in it's entirety. Your question is impossible to answer without more info.Why does it need to be replaced? How many fixtures are in the home? Crawl, slab, or basement? A total house re-pipe can cost from $4000.00 to $12,000.00 on homes in my area(northern Il.) Again, more details would be helpful.
A good way to estimate that would be to count your fixtures . every sink , toilet , shower , bathtub , Water heater ect. If it takes Hot and cold add 300 dollars if it is cold only toilets hosebibbs , add 200 dollars that will get you close . If thats in the ballpark then get a real estimate .