There is generally a prettty hard and fast rule when it comes to how much an agent will charge for a long term rental Marbella. 1 months full rental value is usually the charge you will be levied when using an agent. You also need to be aware that there area also deposits to be paid to the landlord which can range from 1 month all the way up to 12 months.
Be aware that if there is any damage to the property, the landlord can keep hold of a portion or all of the deposit if the work to bring the property up to its pre-rented condition. So in other words, look after the property you are in.