We've been pleasantly surprised at how cheap everything is here in Bolivia, food especially. In Tarija, we went twice to this unassuming place that we wouldn't have found but for the recommendation on the Tarija Wikitravel page, and enjoyed the simple but filling food that included a salad buffet, soup, main and drink for 15Bs (£1.50)!
You sit on plastic chairs at picnic tables, often sharing with friendly strangers who wish you buen provecho (bon appetit) as you tuck in to the home made food brought to you by the only waitress, who despite seeing to about 50 people, is always smiling.

