My advice to your sons would be to share an apartment with some Spanish
people.
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
else.
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.
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.
There is an online agency called Spotahome which looks OK
Click on the link below to check out what they have on offer.
Go to Spotahome