It’s worth taking the time to make your own. Yes, you can just buy assets, but then you will have no understanding of how they work…and when you run into performance issues you will have no clue why this is happening. If you’re unsure whether they are all the same or not, this is a good indication that you do not yet know or understand what is going on “under the hood.” It’s also not really possible for you to be capable of evaluating whether a marketplace landscape material looks ‘worth it’ if you don’t understand what it is that you’re really buying.
Usually when I’m buying an asset, I’m paying for (1) something that I either don’t have the expertise to make or (more commonly) (2) something that will save me a lot of time by acquiring instead of making it myself. Some things are time consuming and it’s worth it to avoid doing it yourself, but you should still know how they work so that you can be an informed purchaser.
UnrealSensei has one on his channel but he also has a video that goes over how to make one from scratch. I recommend going through that before you consider dropping money on assets because the learnings you will gain from doing it yourself are much more valuable than the asset itself.
EDIT: I think this is the one I was thinking of. Unreal Engine Landscape Master Material Tutorial - UE4 Tutorial - YouTube