A DNA test can cost anywhere from around £100 - £350.
Hair samples would probably not be accepted for paternity testing by a reputable lab.Motherless DNA testing ( ie only samples from the prospective father and child) can be carried out, but are considered less accurate and more open to interpretation. Much more accurate is a DNA test on prospective father, mother and child.
Given how open to contamination and thus incorrect interpretations a DNA test can be, all elements of such testing should ideally be carried out by experienced individuals, who fully understand the need to protect against cross contamination.
Paternity testing is a sensitive issue, with potentially life changing consequences for all concerned ; Child, Mother and Father. Better by far to have professional guidance in such issues, such as from a GP.
Consent is definitely required from any reputable lab prior to testing. The signatures of all concerned, and most especially from someone with parental responsibility for the child would need to sign a consent form.