Part Exchange value

I dont understand the logic involved with the values of players after being bought on a P.Exchange.

When you pay for a player with one or more of your own players being part exchanged, the value of him after being aquired is way under half the money you spent.

For example, I bought Micah Richards for £35m with Cristian Ledesma as a part exchange(value only £6m). Yet when I received Richards, instead of having a value of £35m he was only worth £14.5m. Although I wasn't planning on selling him soon anyway, if in the future a club came in for him, I could be losing potentially £20m because of his value.

I can see that if the cash bid is small or the player exchanged is very similar in value to the one being bought that some form of decrease in value can work but in cases like this, clearly common sense is lacking and I find this a small but annoying problem in the game.

Although this was more of a rant than a question, feel free to leave any comments.

Also, sorry if this topic has already been posted but I dont have the time to look through every post until I find it.

thta's how clubs earn.