You need to use a column or row (depending on your spread sheet) and let's call it RetailPrice.
Let's say you have the first price in cell E12, the second in E13, third in E14 etc. Put the RetailPrice column in column F. (as it's beside column E where E12, e13 etc are, although you can use any that you want.)
You need to enter a formula in the first cell that you want to work on. In the case that you want to add 76.5%, this formula will be =(E12*76.8%)+E12 (you must put the equals sign in.)
Now, you have your costs in E12, and your retail price beside it in F12.
To expand this through the entire range, just look in the bottom right of F12 - see the cell corner is darker/bolder? Just hold your cursor over the emboldened corner until it becomes a plus, and then right click. Leaving the button pressed, you will then be able to drag the formula through the entire column.
When you want to change the formula, just change the one in cell F12, and repeat the click and drag.