To be more precise, my understanding is that it's not gross income, but net profit. So, expenses are not taxed.

Also, 92.35% of that profit is taxable, and as dragon writer said you stop paying FICA at $113,700 and then you play only 2.9% for Medicare.

Here are examples of self-employment tax at different levels of income:

    $100,000 - $14,129.55 in taxes, an effective rate of 14.13%
    $200,000 - $19,455.10 in taxes, an effective rate of 9.73%
    $300,000 - $22,133.25 in taxes, an effective rate of 7.38%