What you are describing sounds like melasma. This is a condition where women get dark areas on the face from estrogen hormones AND sun exposure. The estrogens are from pregnancy, birth control pills or even normal internal production of estrogen. Melasma usually consists of large dark "spots" on the face and not small dark spots. Treatments vary and include bleaching agents (hydroquinones), acids (glycolic, salicylic and lactic) and retinoids. Effective improvement usually employs combinations of the above. Treatments is usually ineffective if you don't use sunscreen, SPF 15 or above, twice a day.
As you can see, cost varies. You should be able to get a comprehensive program for well under $100.