I know there are a lot of posts on this subject and I do believe if everyone would read one of the first ones from RDL, you will see how much to donate. Basically he says to donate what the program, Alt.Binz, is WORTH to YOU. If you use that as a guide, I don't think you will have any problems donating.
Like Hecks mentioned, and I have used a lot of different Usenet Readers. My first one was Agent and I paid full price for that one. Then I found Newsbin Pro and paid full price for that one. Newsbin Pro was my main Usenet program for a long time. Got Grabit and Newsleecher, full price for those. But I kept coming back to Newsbin Pro as it was easy to use and did everything (I thought) I wanted it to.
About 6 months ago someone mentioned Alt.Binz on the NewzBin site and I did a Google for it. This was about the time that RDL posted his "The Future of Alt.Binz" message and I thought for sure that was the end of the best Usenet Program I had found. When RDL started the Donation for Alt.Binz, I was eager to donate at least what I had paid for all the others I had purchased, as it was WORTH it to ME.
And ever since then, Alt.Binz has done nothing but get better. I am not too savvy on a lot of the other things that people have requested, but it helps me in the long run by doing things I would have never thought of.
So, I guess all I am trying to say is What is Alt.Binz WORTH to you? If you say 1 to 2€, then you are doing nothing to RDL but slap him in the face. If you say 20 to 30€, then I would say that is a great start to keep the program and RDL AND the others that help out, into continuing to developing the program.
And speaking of Usenet, it was around many, many years before P2P, Torrents, and everything else that tries to share files. And with all the Usenet servers adding SSL to the servers, I really believe it will be around a lot longer than most people believe.
Just my 2€