From fuel prices to tax credits, which hybrid pays you back?

Mercury Mariner Hybrid FWD

Base Price (inc. dest): $28,845.00
Tax Credit: $3,000.00
Adjusted MSRP: $25,845.00
Premium: $3,610.00
MPG: 34/30 (city/hwy)
MPG Benefit: 10
Combined MPG: 32
Annual Gas Cost (15,000 miles, $4.20 per gallon): $1,968.75
Gas Savings: $894.89
Years to Payback: 4.0
EPA Pollution Rating: 8
Compared to: 2008 Mercury Mariner FWD
Base Price (inc. dest): $22,235.00
MPG: 20/26 (city/hwy)
Combined MPG: 22
Our Take: “One of the smarter hybrid deals on the market, combines low emissions, fuel savings and a significant tax credit with SUV room and convenience, but it doesn't pay for itself as quickly as its corporate siblings, the Ford Escape Hybrid and Mazda Tribute Hybrid.”

Details, Price Quotes:
Mercury Mariner Hybrid