Cheers, Carpenter — maybe I'm not being taken for a complete ride, after all! I'm still waiting for his detailed breakdown, so I guess I'll reserve judgement until I see that.
However, I don't think he was planning to cap the walls or render the blockwork — at least, none of the surrounding sites have anything other than bare block walls, most of them pretty DIY-ish-looking
— and they can't be built higher than 1m from the ground because of planning restrictions in the area. So his materials are probably going to cost a bit less than you've reckoned on above — especially since that particular part of Andalusía (Atlantic coast, Cádiz province) is renowned for being
dirt cheap for blocks/tiling/plumbing supplies, etc. I think most of the big builders in Málaga do their shopping over 'our way'.
We'll be going over for a couple of weeks in July — maybe I should start improving my own DIY skills?
(although bricklaying in 30/35° of heat is
not the most tempting way I can think of to spend the hols...!)