Airlines are business. They charge certain fares because they can still get people to fly at that fare. If they can't, they start dropping fares.

If A3 are charging €800 for LCA-TXL-LCA consistently, then they must be able to charge it and still have people buy the ticket at that fare.

Not much good will come from complaining about it - the only thing you can do is to vote with your wallet and go with an airline that can provide you with the service at the price you are willing to pay, as you seem to be doing already.

In terms of the value of flights, personally, I think €390 LCA-TXL-LCA seems ridiculously cheap. €650-700 is perhaps feels more 'normal'. €800 is perhaps a tad expensive but not very expensive...

Pragmatism and not getting annoyed with fares will go a long way towards making one's life more pleasant. I can't imagine getting annoyed at some of the fares I sometimes end up paying - my blood pressure would be sky high.