Balayage is more expensive than highlights for a few reasons.  First, this technique can take longer as the stylist completely customizes where the light pieces should fall and hand paints those.  Once in, a balayage color  is much easier to maintain. The soft painted pieces of a balayage application grow out without a line of demarcation unlike traditional foils.  Balayage can last up to 6 months and for this convenience, it does cost a bit more.  Once someone has balayage color, they may be able to come in between getting the whole balayage color done again and just brighten up the front face framed pieces; this can be less expensive than foils and quick.  All balayage looks are customizable and it is best to talk with your stylist about the plan that is right for your hair.