Just because you think something is expensive does not necessarily mean people agree with you, no matter how much you "explain" your point of view.
There are people who agree with you, there are people who aren't.
The reason cartels are illegal in real life is because there's nothing the common folk can do about it, as it takes education and dedication to properly provide goods or services, but PSO is a game where you're essentially always trading money for time (and/or vice versa). You buy something so you don't have to get it yourself, to save yourself time, and if you disagree with a price, you're essentially saying that the time-sink to get it yourself is much more worth it than buying the item.
A "cartel" in a video game does not matter, because you can take action to deal with it yourself that isn't just telling other people to take action. When it comes to C-Mode specifically, it can actually be done relatively easy with just 2 players if you try, it's not like you need to struggle to find 3 other friends.
And once again, this isn't some conspiracy to keep prices up, it's because we felt like S-Ranks were worth more than 10PDs of our time. If other people feel they should be cheaper, they can just never buy from us, or C-Mode themselves and sell for cheaper.
You also have to remember that to get S-Ranks you need friends, and friends aren't going to compete with each other over prices if they're running together, you know?