Understand that when you sell real estate, you are required to recapture and pay taxes on all depreciation taken. If you did not depreciate the property and it's other listed assets, then you're still required to recapture and pay tax on the depreciation you should have taken. Recaptured depreciation is taxed at the "normal" rates, anywhere from 0% to a maximum of 25%. While your gain on the sale is taxed at the capital gains tax rate. You can look at the SCH D as well as the 4797 to see the breakdown on this.
However, with the large change you mention, it "sounds" to me like you may have put a decimal point in the wrong place, somewhere.
