You will have to live in Spain and know people to get high quality cheap oil. I have my own olive trees and we bring our olives to a small cooperative press; in that community the oil costs next to nothing, however, when it’s exported (and most here is for local production and is sold locally only), it is priced to what you find in the shop.
If Spain is anything like Croatia you are not really poor if you own olive trees - you're probably middle class. Poor people sell that kind of inheritance very fast, and also buy cheap supermarket olive oil.
Land with olive trees is really not worth selling where I am unless it’s huge (and even then; people have 100000s m2 and no one is interested in it; you have to maintain it because of fire hazard and you cannot build on it). If it has a (legal) house on it then, depending on the state (if it’s inherited it will be almost or completely classed as a ruin most likely as it wasn’t kept up properly), it’s worth more, but still not a jackpot.
If you are poor and want to live here you are better of putting a caravan on your land than selling it for peanuts and not being able to do much of anything with that money (maybe 1-2 months rent). At least working the land will pay you something and you can trade olives for other goods (many of my neighbours live like that).
Most land / ruins won’t get sold at all; people move away to the cities for jobs and just forget about it, or, family feud and they cannot agree; the result is the same. Usually the neighbour just uses it as their own ‘until they come back’ (which often is never).