According to the Correos web-site, for the first year it costs €64,38 and for subsequent years €53,36 to hire a PO Box. You can also take them for three months in which case the tarif is a little higher.
One name, extra names (up to five) each cost as follows:
Suscripción Titulares Adicionales (extra names) 2,92 (three months) 10,03(annual)
There is aslo a tariff for:
Derecho de Uso Titulares Adicionales 10,79 (three months) 38,98 (annual)
Don't know what this is - extra key holders? - perhaps someone else can help out here....
There used to be an informative leaflet in the Correos offices about PO Boxes (more informative than what is written on the web site). I considered taking a PO Box a few years ago but decided against it because it appeared that if you didn't empty the box for two weeks they could send the post in it back where it came from.