The cost / instruction count there is probably obfuscated by the built in function.
Make your own - its better anyway.
Mathematical instructions are always less than new landscape layers (because in short, the landacape is a piece of s… 20 year old tech that’s never been updated properly).
If you do similar stuff on a mesh, you already have a lot lower of a cost from adding a full on material attributes blend.
Thats the other thing.
A single texture tells you nothing since you have to have a somewaht decent looking end result.
That involves at the very least an Albedo and a Normal map.
Assuming you cheat and derive roughness off the green channel (so grass and foliage is shiny, other stuff is not),
plus plug in whatver else you need off the albedo, and possibly add in or compute heightmaps off the normal map.
The best way to go about it is probably to create a material function that generates your layer.
Then you blend in those final functions.
Now, assuming you blend in with paint, you incur nothing else on top of it.
The material is painted, the layer blend is interpolated and you get your results (Plus or minus height blend).
Assuming you power a lerp (for each different texture/uv group) based on the slope calculation - then the lerp will always be calculating if it should or should not paint said pixel in said way.
All calcs here are always GFX bound - so newer cards means faster speeds.
But because something has a cost you cant quite quantify due to better tech, doesn’t mean said cost is neglegible, needed, best practice, or the proper thing to have.
Additionally, you are dealing with a situation where even shaving 1ms off the total GFX load means the difference between a viable game and an unplayable one for the lower end tear of gfx.
So anyone working professionally on anytjing will always optimize everything they can optimize (particularly when there is absolutely no visual downside involved).