The beginning isn’t as Costa Rican as you might expect.
I noticed the photo is from the old Mercado Municipal de Artesanías when it was on Ave 2a by the Plaza de la Democracia y de la Abolición del Ejército. It would be interesting to survey Ticos born prior to 1950 or so to see if they remember when they first started using the phrase. I would imagine someone at some University has made it the subject of a thesis in Spanish, Hispanic Culture, or Linguistics. When I studied at Universidad Nacional in Heredia in summer 1992 for my Masters in Spanish from the California State University, I wrote a term paper on Tico slang, contrasting it to slang "argot" in Mexico and Spain. Costa Ricans are pretty bland on their cuss words compared to the Spanish and Mexicans. I included a section on weather terms used to describe rain. The terms are plentiful and abundant in Costa Rican Spanish much like terms for snow and ice in Inuit languages. The use of the term invierno for the the rainy season in what is technically summer in the Northern Hemisphere really blew my mind. I did a survey of multiple slang terms frequently used at the time among host families, students at the university, and any Tico I came into contact with at the time. I don't recall the twrm Pura Vida being as ubiquitous as it is now. Im married to a Tico from the Zona Sur who when I met him spoke in colorful Tico dichos frequently, so I had to learn numerous very countryfied Tico expressions like, "Que sabe un chancho de amor si nunca le han dado un beso, hay mas tiempo que vida, etc. Pura Vida is the least of it.