The entrance ticket of the Alcazaba and the castle of Gibralfaro are very cheap in Malaga but if you are so lucky to visit Malaga on Sunday you will enter in these places for free! Sunday is the free entry day of the Alcazaba, and Gibralfaro starting from 14:00.
Malaga has a wide variety of museums (36 museums) most of which are located in the historic center.
On Sunday the majority of them have free entry. Here you can find a list of them; if you want more detailed information about the entry time, how to reach them and some useful tips, check this blog post.