The answer to your question depends on the answer to another question: Is the payment like a bribe or not?
Some journals have publication costs or page costs. So In addition to passing the review process a fee must be paid to help defray the journal's publication costs, which can be justified when publication costs aren't adequately covered by subscriptions. Such publication costs may even be allowed for in the budget of a research grant. None of this is per se unethical. However, if one bribes the editor or if the journal's publication fee is excessive relative to the journal's reasonable expenses, that would indeed be unethical.
Maybe researchers with big grants have an unfair advantage generally, but that is a separate issue.
Nowadays there are a number of fraudulent journals soley in the business of extracting publication fees and whose reviewing processes are a sham. While the purveyors of those journals may be unethical, the people who submit articles to them and pay their high publication costs are not unethical but foolish.
In academic writing, paying a journal to publish a research article is considered highly unethical. Journals indexed in Scopus, dblp Computer Science Bibliography; Inspec and the like expect you to upload the article in your area of interest. If the editor finds that the quality of the article matches the standards set by the journal, the article is sent for review else rejected.
The answer to your question depends on the answer to another question: Is the payment like a bribe or not?
Some journals have publication costs or page costs. So In addition to passing the review process a fee must be paid to help defray the journal's publication costs, which can be justified when publication costs aren't adequately covered by subscriptions. Such publication costs may even be allowed for in the budget of a research grant. None of this is per se unethical. However, if one bribes the editor or if the journal's publication fee is excessive relative to the journal's reasonable expenses, that would indeed be unethical.
Maybe researchers with big grants have an unfair advantage generally, but that is a separate issue.
Nowadays there are a number of fraudulent journals soley in the business of extracting publication fees and whose reviewing processes are a sham. While the purveyors of those journals may be unethical, the people who submit articles to them and pay their high publication costs are not unethical but foolish.
While there may be costs associated with the publishing of journal articles it would definitely be unethical to pay fees to an editor (bribe) to get an article published. However, substantial fees upfront to out right publish an article should be looked at carefully due to the high likelihood of that particular company being a scam. A researcher should always investigate the source of an offer for publishing. The number of fraudulent journals are increasing and call for researchers to pay closer attention to which publishers they choose to submit their work. But I would have to agree with Karl Pfeifer, while those publishers who prey on researchers and charge outrageous fees are unethical, the researcher in this case would just be a little foolish for not doing the background investigation into the company and fees.