Francisco,

There is not a right or wrong answer to this question. I recommend you read this guide on property management fees.

http://www.managemyproperty.com/knowledgebase/property-manag…

I don't think there is one answer to this question. I will give you examples of what I pay:
I own a beach cottage in FL. The company that rents, manages (since I am in NJ) They receive 20%. They handle the rental end and also hire the cleaning service and oversees any maintenance problems.
I also have a multi-use building in NJ. One of my tenants manages the building for a rent reduction of $300/mo. He not only manages, but does the routine maintenance.
Find someone you can trust and ask other landlords around in your neighborhood what the going rate is.

This could all be negotiable on type of rental that you have, such as a 4-plex or condo. Obviously a Manager will have more to look after and more liability in a single family house. A condo especially one maintained by an association is much less stress than the usual home. I managed over 700 rentals for 3 years and personally would just negotiate with each client. You may also look for a discount in bulk, multiple units=lower percentage. Newer homes are less of a worry producing less tennent/ upkeep problems as well.