When you sell rental property you are required to recapture all prior depreciation taken and pay taxes on it. If you did not depreciate the property, then you still have to recapture the depreciation you "should" have taken and pay taxes on it. That's your "penalty" for not having depreciated the property as required by law.
The recaptured depreciation is added to your sales price, and your gain is figured from that. Then your carry over losses are subtracted from that gain, to determine the taxable amount of the gain.
Sounds to me like you had around $54K of depreciation that was recaptured. Not unrealistic based on the data you've provided.
