It depends on your other income. If the $300 is your only self-employment income for the year, you will not have to pay any self-employment tax. But you will have to pay income tax on the $300. How much income tax you have to pay depends on what tax bracket your other income puts you in. For example, if you are in the 25% bracket the tax will be $75 (25% of $300). If you receive Social Security benefits, the additional income could make more of your Social Security taxable, which would add to the amount of tax you have to pay.
If the $300 of self-employment income is your only income of any kind for the year, you will not have to pay any tax and you do not even have to file a tax return, unless you received the advance premium tax credit for health insurance.
