Your cost is the cost to produce them. Since that is going to be very difficult to prove to the IRS satisfaction, you should probably assume that your taxable profit is the entire sales price or net value when your mined them.
The problem of calculating cost is how do you allocate the cost of electricity for computing resources if you don't have a separate electric meter? How do you prove to the IRS satisfaction that the amount of electricity you say you spent to produce the coins is correct? You generally won't be able to deduct the expense of your computer or Internet connection unless you have a dedicated computer and dedicated Internet connection just for the purpose of mining cryptocurrency. Then, you would have to file a schedule C and report this as earned income, like a self employment job or independent contractor. Then it would be easier to deduct your supplies and a dedicated mining computer, but it would also subject you to self-employment tax on the net profit of your business. If you simply report this as other income, and don't try to deduct your expenses, then it is taxed as short-term capital gains which is equal to the ordinary income tax rate, but does not subject you to additional self-employment tax.
