Assuming you have an asphalt shingle roof, check the prior comments in the Home > Roofing link right under your question for a number of responses on cost, types of shingles, ridge vents, insulation, ventilation, etc.
Ballpark reroof number - $5-7/Sf usually with normal mid-range shingles.
Typical life for a shingle roof should be about 15-25 years with economy (20 year) shingles, 25-35 with mid-range (25 year), and 30-40 with high-end long-life shingles, barring wind or hail damage or significant organic materials buildup or moss growth. Would typically be 3-7 years less in each case if in a very hot sunny year-around environment like the Southwest from Oklahoma and Central Texas through southern New Mexico and on through Arizona and Nevada to the West Coast, as well as in southern Florida (roughly Miami and south), and at very high elevations (above about 7000 feet) where hot days and cold nights combine with higher ultraviolet radiation to degrade the asphalt faster.
And a pre-emptive answer - no, the shingle color does not make a lot of difference as that only makes a 10-20 degree roof surface temperature variation, though double-layer shingles do generally last longer than single layer.