Cheap by EU standards. I'd give them a call - the numbers are local pilot books. Most are run by the local councils.
We stayed in Las Palmas Gran Canaria last November, if I recall correctly around 35 eu per night power and water included for a 46 footer.
I think I paid 40 euro a night for a 58 footer last December in Grand Canaria, but you had better reserve now if you are planning to be there at ARC time. Lanzarote is also an option, and I paid 600 euro/mo at Puerto Calero for a 45 footer last year.
No - not comparable to most popular Portuguse ports.
As stated before, most Canarian ports have council run marinas which are cheap by EU standards. If you pay 30 euros per night in Las Palmas, expect 60 euros or more in Lagos or Cascaise.