Hi.
I was recently thinking about the incredibly low cost efficency one gets from upgrading high runes to even higher runes ( 3 x 10 > 3 x 11 ) and I calculated the actual benefit one gets from that.
Doing so, I realized how easy the formula I found actually is, so I thought I'd just share it with you.
After a break of a few years I'm still pretty "new" to the game and most propably for many of you this is just everyday stuff, but maybe a few of you could be interested in this, so here it goes:

!! This is for offensive runes !!

FORMULA: (( R(N)  R(O) ) *100)/(R(O) + 30) = Damage Increase [%]
> R(N) = Runes (New): The sum of all of your new runes' level
> R(O) = Runes (Old) : The sum of all of your old runes' level
You could of cause call them R(a) and R(b) or R(t1) and R(t2) as well, but "old" and "new" seemed the easiest to me.
EXAMPLE: I had 2 offensive runes level 4 and one level 5. I recently got 3 level 6 runes. In this case R(O) would be (4+4+5)= 13, and R(N) would be (6+6+6) = 18. Now we put these numbers into the formula:
(( 18  13 ) * 100)/ (13+30) = (5*100)/ 43 =
11,6 [%]
So by increasing our runes from 445 to 666 we get a damage
increase of about 11,6 %.
!!
Note that this is the INCREASE of damage in COMPARISON to before, no total damage increase by runes, which you can of cause read out of the game.
POSSIBLE APPLICATION: I can imagine two: A: You want to find out how much more damage those just spend 30 euros in crystal chips actually give to your character or B: To deliberate whether to buy runes or not, e.g.: Is the damage increase worth the money. Here you could inteligently combine this formula with the calculator that tells you how much money you need for what runes:
http://kampai.libra.uberspace.de/runes/
 This is of cause not my work and I dont mean to constitute it as if, thanks to the author

A Third application would be to find out what runes you need in order to get X% more damage. You can use the formula to calculate that as well:
Lets say you got 444 in off. Runes and you want 10 % more damage. So you insert 12 for R(0) and 10 for Damage increase on the right side and calculate it out. In this case you will get: R(N) = 16,2. This means that if you have 3 level 4 runes in offense and you want a damage boost of 10 %, then what you need is 3 runes, whose level sum is 16 ( or 17 if its nessecary to really get over 10%). So the easiest way to do so is upgrade your runes to 556 ( or 566, cause for 556 you get only 9.5 % ).
Hope I could help or will help someone and if there is anything you dislike about this, go ahead and tell me, maybe the mathematics have completely changed and this whole thing is just nonsense, cause the rune damage increase is no simple damage multiplicator that is untouched by other effects anymore..
See you ingame, Sillphan/Nerzos