My supervisor appoint me as a sub reviewer of one conference. Am I paid for this? Should I tell him or negotiate about the cost of reviewing? I have finished my masters program and defended my thesis.
You will not be paid in money.
You might be paid in reputation. This depends on how the conference handles its review process and you can ask your supervisor about it.
- If the conference organizers learn about your reviewing (i.e. if "being appointed as a sub-reviewer" is more than your supervisor forwarding you the manuscript and asking you to write the review that he or she will then send back to the conference organizers), that may lead to a slight rise in your reputation among those organizers. Results could be that you will at first be asked to review more papers, but on the long run, this might lead to you being considered for e.g. co-organizing a workshop (if one of the organizers remembers you usually provided valid input in a particular field etc.).
- Some conferences publish their list of reviewers. This has basically the same effect as above, however with a much wider scope, as the fact that you contributed will be publicly visible.
And of course, you will gain some experience in reviewing and more awareness of issues in authoring scientific texts. Also, you might get some insight into the work of researchers that you may not otherwise have found. Sometimes, this includes becoming aware of research that you can use for your own publications (either as related work or as an inspiration in the first place), as seems to have happened e.g. here.