This is a difficult route.
The only cheap direct way between Marbella and Porto is by bus via Seville and Lisbon.
For the bus:
There is a bus departure from Marbella to Seville(Prado) at 16:20 daily. Transit time 3.30 minutes. You can then connect with the alsa.es departure from Seville(Prado) at 23:45 arriving Porto at 12:15.(Portuguese time) the next day.
Alternatively, you could take a train from Malaga to Madrid and then a low-cost flight (ryanair.com) to Porto from Madrid to Porto.
By bus/air:
There is an alsa.es bus departure at 08:30 from Marbella arriving Madrid (Estacion Sur) at 15:45. You could then take the Metro to Madrid Barajas Airport to connect with the ryanair.com departure at 18:40 arriving Porto at 18:55 (Portugal Time.)
