My advice to your sons would be to share an apartment with some Spanish
1. It is the cheapest method. 2. You will learn more Spanish and get
to know more people.
Make sure that you really are sharing the apartment ie each person
pays equal parts of the rent and not you paying more than everyone
A student apartment for about 4 people would be about 500 euros pm
The best way of finding a cheap place to stay is to go to the universities and look at the notice boards. There are always hundreds of notices advertising flat sharing. The price of sharing a flat can be very low. Just go to the universities and note down the numbers and then phone to arrange to see it.
If your level of Spanish is very limited then you could put up your own notice that the translation faculty
because translation students are always happy to share with foreigners because they can practice their English.
The second best option is to look in the Ideal or the Almoneda newspaper. Beware of agencies which offer to find you accommodation by paying a fee. I have heard several reports that they are a are rip off.
For when you are looking a good place is Jose's apartment.
The price is cheap and it has 2 bedrooms and 2 bathrooms.