This can vary depending on where you go (e.g market, salon, supermarket...), and what you want the actual extensions to be made of (e.g human hair, animal hair, synthetic materials...) It also varies...
That depends on the operator, the type of extensions and the Quality of the hair. You need to visit a number of salons and get Quotes and information...take your time to decide
Many are $100-300 but this online store is REALLY cheap. For hair extensions anyway. It all depends on the length of your hair, what kind of hair extensions you want.. Etc..
If you live in Massachusetts it is free because the president thought that since Ben Franklin lived in MA they are awesome which really isn't fair cause I live in Australia mate!!!