I really do not know what you are trying to do but you need to compair apples to apples.
You looked up D1NL6600A at o'reilly and they crossed it to M67E with a price of $59.99
Looked on this site for a oil pump and saw their price was $133.54
While both pumps are made my Melling they are not the same. This site sells pump C3NN6621A that crosses to M67B. O'Reilly can get M67B also but it is $90.99
Now if you want to complain about the mark up from $91 to $134 then you are right. This site did the leg work crossing the pump.
Melling is a good product so buy where it is cheapest.