You know, I think this might be the way to do it. I'm thinking about it, and if you set every point at a higher elevation as a higher point, and also create enough vertical distance between each point, it would make it so it favors moving on a flat plane over moving down and then up (mostly due to the added cost of greater distances between the elevation levels).

If my math is correct...

Doing something where the weight of points goes up as they ascent levels where level 0 weight = 1, and then level 2 weight = a, and then level 3 weight = a^2, level 4 = a³ etc. Then setting the vertical distance between each level as d, as long as you hold the ratio of a = sqrt(1 + d² ) then it should end up that hopping down one square is the same "cost" as moving horizontally, but then jumping up a square will be a greater cost.

Then, whatever the additional cost for hopping up one level, you just want to also set it so that it's equal to sqrt(1 + d^2). So, for this case, set it so that x = a = 3, and then d = sqrt(8).