What would Marx see as the basis for people's ideas and values?
To Karl Marx, the basic cause of people's ideas and values is the "base" of their economy. To Marx, everything about a society other than its "base" is a "superstructure" that is built on and depends upon that base.
A society's base is, to Marx, a set of economic factors. The base is made up of things like the relationship between the owners of the means of production and those who do the work. This relationship, along with things like the way in which labor is divided up, is what forms the basis of the culture.
Therefore, people's ideas and values will be formed by the base and by their position within that set of relationships. A person who is from the working class will have different ideas and values, for example, than one from the class of property owners.