Facing problem Unit Of Measure and conversion factor

I purchase the keyboards in boxes and each box has 3 keyboards within it. The default measure of the unit is Box. I set the purchase and sale price of the Box at the time of item creation.
I purchase the keyboards in boxes and sell them individually using NOS as a UOM.
Now the question is how can I set the conversion factor when I sell a single Keyboard and it fetches the sale price of a single keyboard, for example, the selling price of a whole Box of keyboards is 30000 and when I sell a single keyboard and system automatically fetch 10000 sale price of single keyboard, is this possible? or I have to manually set the selling price.