If my product costs me $1.71 to make and I sell it for $3.50, what is my profit in a percentage?

Your profit as a percent of revenue is


\frac {Revenue - Costs}{Revenue}


Convert this to a percentage. Actually this is better described as gross margin, as the only cost included is cost of goods sold. To find profits you have to take into accouint other expenses such as selling, general and administrative expenses, and other fixed costs.