Conditional formatting Problem! [Excel 2011 MAC]

So what I'm trying to do Is creating a new cell result in which i calculte a value. This value is the difference between the values of Cell G2 - (The sum of the values which names are turned green)
How do i do to automatically get all the values that have green names?

Or just creating a function using VLOOKUP or something that subtracts G2 with all the prices that have equal amount and available times its amount.