^^What she said. The amount of hair you buy also depends on the size of your head, how long the braids will be, how small the micros will be, etc. Will you use synethic or human? This plays a factor in price.
Some people have allergic reactions to some synthetic hair, such as an itchy or sore scalp, so that could make a difference in the type of hair and the price you may pay. Two synthetic types that come to mind are kankelon and toyakelon.