Hi Woolli,
I paid €116 euro's for 2 of us as foot passengers - this was with an upgrade to their gold service .... I think it was €90 for standard service (gold = just better seats and free drinks I think). Fred Olsen is a fast cat ship and only takes 1 hour and there is a free bus to take you from the port in G.C. to Las Palmas (about 20mins).
F.O. Ferries leave about every 3 hours. So yes you could easily leave early and get back late the same day.
Yes we could have taken car but decided to use the free bus as we are only staying 1 night for a short break so will stay in Las Palmas this time.
http://www.fredolsen.es/lineas/english/index.asp There is the Armas ferry service which takes about 2-3 hours and they quoted €43 for both of us. Bear in mind we are residents of Tenerife and get a discount off the normal prices ( I think about 40% but not sure about that).
http://www.naviera-armas.com/ Hope that helps