If you are using a data validation list with only A, B, and C, then you don't really need the error check. And if you fix the extra space, you could use a simplified version.

Although the other members' formulas work every bit as well, I had a very specific reason for writing it the way I did. The fact that 250 is 2*125 may be a coincidence, and I avoid taking advantage of coincidences in programming logic. However, I will admit that the fact that B and C correspond the same value may also be a coincidence. So the most loosely coupled formula is

Formula: copy to clipboard 
=D2+IF(E2="A",250,IF(E2="B",125, IF(E2="C",125)))


This makes it obvious what is happening and which value corresponds to each of A, B, and C. This way if any of the values change later, it is very easy to update the formula. It is not elegant, compact, or clever, but it is very easy to read, understand, and update.

(P.S. It is not necessary to quote the entire post that you are responding to. In this case you didn't really need to quote at all. We have a "rule" about that but it's really more of a guideline.)