The classic discussion.
In my humble opinion, there is a difference between what you think that an item should worth and what an item worths. Even thou, someone thinks a item worths x, that doesn't mean that it is the price that is sold.
First of all, remember that evaluating an item is really subjective. For example, some people would say that a rolex watch (which costs between 10k-25k euros in my country) its a stupid and unresonable price for it, but they actually sell alot and are one of the most succeded brands in the world. In the end its just a watch with some fancy materials and rocks. You can buy a good watch for 100 euros that does the same damn thing; see the hours. But some people really desire to have fancy rocks and good steel, or for social promotion to show to everyone, so they pay like 20k for a watch that does exacly the same thing that a 100 euros one (not to mention that 20k in my country its a price of a good car, like a brand new BMW). That happens with PGF too. Its not that people actually NEED a DW, they just want one, they desire them, to show off to other people or to just make the game less effort or just to have fun with the unique features that a dark meteor or a dark flow give you. I personally bought my DF for 400 DTs once, just because i love the unique mechanics that the weapon has. And i really don't regret about it, for real. I simply didn't need it, i just desire to have one. So if you really desire to have a rolex, you will pay the price of a brand new BMW, even thou you will not have problems to buy one, instead of a car, if you have the money ofc. So what price is not correct? The Rolex watch price, or the BMW price? Or both are correct? Think about all the useless crap you bought in your life, was the price worth it? Even thou, you bought it.
Second, look at the rates. Last year i got 2 PGFs, kept one for me and gave the other to a friend. I think i don't even have to talk about rates (but i'll bitch anyways, they were stupiditly unresonable last year). The same thing happened with STA this year. STA was a damn unresonable rate, that took me "years" to make one drop. So, if you suffer and work damn hard to get your drops, how can you sell it for cheap? If i suffered to get my PGFs and STA, why would i sell it for a price that doesn't even reflect the effort i put it on it? And thats when subjectivity enters; for me, for my effort, for my damn hard work, for all the hours i wasted playing a videogame, i wouldn't sell my PGF/STA for less than 1242353463564563 DTs, but other person, maybe got lucky or just don't gives a shit about dark weapons, wouldn't mind to sell it for 100 DTs. If there is not regulation in market and never was, how can someone say that 350 DTs is a wrong price for it? If the market is liberal, how would you give the exact price of something? What variables would you use to say: "ok i did the mat and the exact price of PGF is =x"? You can't, so the market and how much people desire an item, is the price of the item. Sad or not, it is what it is. Not to even talk about how DT system is expensive.
Third, its not just because the greedy people, its because the fact you can only have it 3 weeks per month and have absolute stupid high rates to final bosses. That doesn't motivate people to sell their hard earned item without a price compensation. There is nothing dirty about it, you worked for it, you should be rewarded in a way or another. That's how life is.
There are no miracles. Saying that its just greedy people that made this, is really reducing the high price phenomenon. Its looking to the sea's horizont and thinking that there is no more land in the other side, just because you can't see land.
As i stated before, that's just my humble opinion, no absolute truth was written. Buy for the price that you feel @Ana G.5 that expresses the desire to have the item and no one should tell you that something worths this or that. Good luck, hope you get one