10% cost is easily put down to overhead/ labor/ energy the station spends to remove old module and install new. No one works for free.
I guess if you get a bigger gas tank put on your car and decide you don't like it you expect the mechanic to donate their time and take a hit on selling a now used part? 10% fee is realistic and in some cases may be generous if the module has been in use for a long time.
Indeed, we can all come up with "real world" reasons why modules should cost 10% to sell. But this is a game. How does people losing 10% on modules improve the game?
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