South of Spain will set you back �300 for a month - depends on what you like. Have a look at the accommodation offers in some of the newspapers travel sections as they often have long stay offers.
Also if you want to make your own mind up as to a location, have a look at
http://www.andalucia.com which has tons of info about the area - suggest this as good as any as at least you have some other options to enjoy like Granada, Seville, Toledo as well as the coast - not just one island attractions like Tenerife......and prices can be pretty good as well have having facilities open, not like some strictly seasonal places...