Well, tell us which pilgrim meals or restaurants along the Camino that serve dishes that consist of a heap of all kinds of sauteed vegetables in a big plate, which is ubiquitous in Thailand/Vietnam at few dollars? No, potatoes do not count.
Firstly, there isn't a "the Camino", there are many Caminos across Spain. This is, I think, key to understanding your options when you walk towards Santiago in Spain.
I am loath to compare prices between Spain and Thailand/Vietnam because there are many variables and so I will ignore that aspect of your question.
Somewhere above your question someone posted an explanation of some of the cultural aspects of different regions of Spain. The potato holds a special place in the cuisine of some areas in Spain and beyond that during relatively recent history the Spanish government at that time passed laws that required a "menu del día" that was required to be offered at a relatively low price.
Those laws are no longer in place but they created a tradition that many small restaurants still follow because their customers often requested it. From this concept many small restaurants that often served pilgrims also developed a cheaper version that they call a pilgrim menu or something similar because their customers requested it.
Given the requirements for low cost most of these menus feature a lot of carbohydrates because carbs are cheap and provide energy. Potatoes, beans and other cheap vegetables usually feature, depending on the region.
Of course, if you and other pilgrims on the
Camino Frances want to look further afield you will find a greater variety of food at a variety of prices.
I posted some photos earlier in the thread, mostly in a spirit of fun but all those dishes were purchased in places that are part of established
Camino de Santiago paths and some of them are from the
Camino Frances just a month or two ago.
The food that you desire is available if you want to go look for it, it may not be at Thailand or Vietnam prices but Spain isn't Thailand or Vietnam.
If you are interested in the tradition of menu del día then look on the Internet and here is a couple of random ones (via Google search) to get you going.
Conceived by a dictator, a tradition that cheaply opens up local eating opportunities
blairmastbaum.substack.com
Franco did a lot of terrible things, to be sure, but one thing he did right: create the Menu del Dia. In 1964, he designed a set-price menu…
medium.com
The menu del dia is the most economical way to eat the large midday meal in Spain. It has three generous courses for a single price, including drinks.
www.thespruceeats.com