Hello Marianne,

I am not sure if you want to know if author should pay or if you are asking about the worth. If it is about the worth, then I think that is very subjective. Do you feel it is worth paying for or not at it would be for any open access journal. But, if you want to know  if you should pay or not, then it depends on the Journal policy. Once again, if it is open access, there is a higher chance that you will pay for the article's publication it is not an open access, the higher chances are that you will not pay.

Hope this could help?

Thank you for your answer, but my question is about the ethics in payment

Will it be a differtial approach for authors submitting a paper directly and in response on the invitation respectively ?

I very often receive invitation from editors to publish articles. However, most of time they charge fees for those articles. It goes without saying that I never submit papers to such kind invitations. There is, as it happens, a double moral in here. 

No

If the publisher can't find a sponsor, then he/she has to charge. However, sponsors can be found, but they have their rules as well. One has to compromise and adjust rules to produce publications that do not charge authors.