There are some schemes which allow to save money if it's the very first time that you register as such. But they are only a short- to mid-term solution (covering typically the first year), but in the long term (starting from the 12th to 18th month) you'll end up paying the full quota.

If you can read spanish, articles such as this one explain the options: http://gestron.es/ventajas-ser-autonomo-trabajar-cuenta-ajen...