You should expect to pay self-employment taxes of 15.3% PLUS ordinary income taxes (whatever your marginal (top) tax bracket is, if you have other sources of income). For me, that's a nice, whopping, 48.8% (SE + Fed + local). You won't pay any tax on expenses, as mentioned (treats, toys, transportation, advertising...anything you ACTUALLY spent running the business). You also won't pay the full self-employment tax if your income otherwise is over $118,500 (you don't owe social security taxes on income above that number...I'm not sure how that works if you're married filing jointly). You may be eligible for other deductions or credits.
Because I already use an accountant, I just told him that I was doing this and he told me how much to set aside (as a % of earnings). If you don't use an accountant who you can easily consult, you probably want to set aside at least state/local and federal taxes at your top tax bracket and the 15.3% self-employment tax. If you do that, worst case scenario, you owe less and can give yourself a bonus!