So both magic armor and magic weapons have the text:
"Runes of +2 armor/weapon potency or stronger require the armor/weapon to already have the listed weaker rune, and etching the new rune increases the existing potency rune to the new value."
As I read this, by RAW, to add a +3 armor rune to light armor doesn't actually cost 730 gp as indicated for the +3 rune, but is actually 730 (+3 rune) + 300 gp (+2 rune) + 25 gp (+1 light armor rune) = 1055 gp?
Why? Just have the actual net cost with each rune and you just apply that rune in place of existing runes~
Probably to accommodate for the instances where somebody will have +1 rune and wants to upgrade to a +2, they just look in the book and pay the price listed, instead of subtracting the cost of the previous runes from the listed price.
Ex. I have +1 wep, want to upgrade it, pay 300gp, now have +2.
vs
I have +1 wep, want to upgrade it, cost of +2 rune is 325 - 25, so 300, now i have +2.
It's probably more common to upgrade something by 1 step than it is to go from 0-3 so they're providing info for the most common uses.