We allways give 3 prices for each mod: Low, Medium and High.
What price should you look at?
Medium is what seems to be a good and fair price for both.
Low is for desperate Sellers that really need the Platinum now and don´t care much about the mod they are giving away.
High is for Buyers that are really looking for one special mod that they really want.
Depending on who wants what and how much a good price that makes both Traders happy can have a wide range, just take our numbers as a suggestion to avoid making bad trades.
And never forget: Warframe is about Fun and Combat, not so much about haggling for prices all day!