We have found that it very much depends on location and type of property.
i.e. small studios in areas frequented by students might attract 1-2 students, where a slightly larger 2 room 1bed apartment might attract 4 students who are looking to reduce their personal contribution to the monthly rent, but combined, works out % wise better for you...
Managment companies can typically charge up to 15% of the rental (or just over 1 months' rent) but if you deal with a good one they will look after absolutely everything including tax returns etc that are payable on your rent.
We have dealt with a number of agents/managers in different areas and have found that if you go the right way at the start you should be ok.