Unlike the property market in countries like The United States and Sweden the Spanish market is not transparent hence it makes it more complicated buying a new apartment or villa. In the U.S. you can on sites like Trulia find information about the average price that a property with a certain postal code is listed and sold for. This makes it easier for you to assess if you are making a good investment or not.
Here are some figures for the different areas in and around Marbella:

Sold detached villas Average price (€) Price per m2 (Built)
Nueva Andalucia € 1,373,903 € 3,673
The Golden Mile € 1,860,660 € 4,041
Benahavis € 1,532,532 € 3,005
The New Golden Mile € 753,369 €2,562
San Pedro de Alcantara € 949,030 € 2,603
Marbella € 873,720 € 2,459


Source: Resales