about academic proof-reading, what to do in this situation? I have submitted an article for a journal in the field of CS and it has been returned and suggested to pass thru a professional editor or proof-reading (this is because my mother language is not English). So far what I did was to pass my article to some online paid tools such as Grammarly and Paper Rater, checking the suggestions, lifted the corrections made by these programs (and being careful enough to check it first with some English grammar books). After that I sent the new version to a past Professor from my master studies and he returned me back the paper with some corrections (he is an English native speaker, but he told me that he was not a professional editor so maybe there could be some mistakes along the way). With all these changes I submitted again the paper, which by the way in the reviews that I got it was accepted by three reviewers, but one of them mentioned that the English needed more work. So I waited, and again it was rejected by the editor pointing me that still the English part needs to be corrected. 

So what to do? I have found that there are some proof editing services online, but how to know if they are good? I have not seen the reviews of any of them and well the prices are not so cheap also (ranging from USD 350 to even USD 700). The services that I found were:

Enago, https://www.enago.com

Elsevier, https://webshop.elsevier.com/languageservices/languageediting/pages/howdoesitwork.html

American Journal Experts, https://secure.aje.com/en/prices

Wiley, https://secure.wileyeditingservices.com/en/prices/quote/new (this one its pricing form looks mysteriously the same as the AJS)

Any advice what to do in a situation like this? I would not like to submit to another journal because it passed quite a long time since this research started.
I don't have any advice about who to use, but I do suggest that you look to the long term. Try to find a person or a service that you will be able to work with on future papers, not just the current one. The person will get used to you and how you write and so the task will go quicker and maybe cheaper in the future. 

Don't look for the cheapest service, I think. Your English skills will improve over time, but you will probably have a better experience if you can establish a relationship. You might even get writing tips from such an editor if you ask, after the person gets some familiarity with how you naturally write.