Under eye darkness can be due to multiple causes like pigmentation or redness( due to blood vessels) or hollowness ( called "tear trough). Treatment depends on the cause and once we know the exact problem treatment plan can be made. For pigmentation there are different creams that can help  apart from that we can do peels and various other under eye procedures. For redness and pigmentation both, Laser is the best treatment. For hollowness under eyes we can inject hyaluronic acid fillers with which you can notice results immediately and it lasts for more than a year. Treatment costs depends on type of treatment you need and it also depends on area, qualification and experience of treating doctor.