When you bought this property, was $200,000 just what you paid for the land only?
Basically your gain here is going to be $300,000. That is taking the selling price of $400,000 less your cost of $100,000 (half the land's original cost). Your gain would be classified as a long term capital gain and that tax rate is currently capped at 15%.
Sometimes if you are in a low enough tax bracket the some of the gain can actually be tax free, but your income level just about brings you to the point where that is not something that would apply. So basically you are looking at a federal tax bill of $45,000 on the sale.
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