We should be talking units you consider good. The argument is that skinks aren't good unbuffed, so we should be able to find "plenty" of units that you consider good, that are just as wound efficient as skinks while doing more damage.
Your math was also a little off on horrors. 40 horrors do an average of 10 damage against a 4+ save. Skinks do 13-14, assuming no reroll 1's. But if your comparison builds the entire cost of the starpriest into the equation, it really should include that which increases skink damage to 15-16. 5 extra damage is not insignificant, especially when a good chunk of that damage is mortal wounds, so skinks are only getting better as you shoot tougher stuff.
You also can't take blue horrors in units of 40 unless they popped from a pink unit of 20 (which would make them cost more). Meaning their +1 hit is disappearing real fast and their range is a ton more restrictive.
IMO that kind of stuff thats hard to just math out makes a big different on the tabletop.
aright now that life has stoped kicking me in the nuts lets do this. first we have to establish that points per wound is a bad measure of what is good in aos. not hard, see the following (10 man 1 wound per man save 6+ 60 points) or (10 man 1 wound per man save 4+ 60 points). now obviously the second one is better same points per wound but with a better save this is why we use "effective points per wound" and account for the save. even this isn't perfect as (10 man 1 wound per man save 4+ 1000 points) and (10 man 1 wound per man save 6+ 5++ 100 points) and (5 man 2 wound per man save 4+ 100 points) all have the same effective points per wound but the second two are better then the first due to the rend mortal and battle shock resistance. this is why all my mathhammer unit break downs have full graphs for each unit but as this dissertation is going to be long enough as is we will be using 0 rend attacks coming from a 4+ save unit and expanding when necessary.
second we must acount for buff heroes now there are two main types of support heroes auras and 1 offs. to have the unit performing it's best you HAVE to pay for the heroe so the points have to be added into calculations. one offs are easy as they can only support one unit at a time and so the cost is just added to gether for every one and we call it square. even if they can go support someone else the universal nature of this will keep things more or less fair regardless. aura heroes are more tricky as they can support more then one unit at the same time, to keep things short we will divide wholey within auras points by 2 and within auras by 3. again keeping things short.
so keeping that in mind skinks with shield have a EPPW (effective points per wound) of 4.5 and non shield skinks have a EPPW of 5.14. this will bee our baseline for durability on units from hear on out. unbuffed bolt spitter skinks have a EPPD (effective points per damage) of 108 in small form and 54 when in horde. both range and melee are the same. buffed skinks have a EPPD of 150 in small and 32.25 in horde. this will be our base line for offence
Eternal guard are one of my favorite low key defense units not the best thing in their army but still solid. they have a EPPW of 4 when unsupported and not moving and a EPPW of 0.78 when supported. now this is deceptive as they are melee units and must remain stationary but still solid OB holders.
they have a EPPD of 36 when moving and 27 when not. this makes them a more effective screen then skinks as you get more for your money both defensively even unbuffed and they can hit back much harder(granted with no mortal output). they are slower then skinks only 6" move but with +1 to run they are only down 1" when used as a screen and their army has a teleport as well.
demigryph knights. with a EPPW of 4.44 they stand as durable as shield skinks are. they have a EPPD of 23.42 with halberds unsupported, 28.24 with non charge lances unsupported and 17.46 with lances on the charge unsupported. with support this jumps to 14 for lances on the charge (halberds get worse). they move 10 and +1 to runs and charges. still a melee unit but they are also considered to be a bit to expensive.
Iron drakes this one gets kind of complicated so bear with me. iron drakes have a EPPW of 7.5 when hit with melee and 5 when hit by ranged unbuffed it's over all much higher but not overly so when hit from range and better then unshielded skinks. now their EPPD when unsupported and moving is 44.07 when stationary it's 22.01 now buffing them makes them even more fun with a stationary EPPD of 14.22 with what is essentially mortal wounds. they are slower but they come from a army with solid teleports and then you just have to ask if less durability is worth 2 times the damage and taking 60-120 wounds out of your opponent turn 1.
Kurnoth hunters(with bows) EPPW they teleport to move so are always planted 4.22. they also buff themselves so their EPPD is 53.44 at twice the range unsupported. not the best unit out there but needs no support to function and is better then base skinks the good warscroll we are working from.
warplock jezzails best non kroak hero snippers in the game EPPW 20 in melee(ew) 7.78 at range better. EPPD is 45.16 all mortal wounds or rend -2 they teleport to move and need no support to work. not as durable but you have to eat threw a entire scaven army to get to them and better damage then anything out side a 40 man buffed skink block.
i'll come up with more but thats it for tonight im board
i have not found a 1 to 1 skink parallel yet but i have only touched on 3 armies so far(maybe ungors? i'll have to check) and there are more to come. what we have seen so far is that skinks are not the most cost effective screen in the game not by a long shot with both eternal guard and demigryphs doing it better both being more cost effective, both doing much more damage when charged and one being faster while the other is just barely slower. nether of them are considered to be the best units in their book, skinks are out side of kroak. damage wise skinks are far out classed by almost any good ranged unit with the ones i put above ether being massively more damage wile only giving up some durability or solidly comparable while not needing support at all to do so. all of the above significantly out class base skinks which was the original contention.
initial hypothesis skinks are good but not broken and their bass warscroll is rather $&*$. conclusion more data needed
skinks are generalists the only time they do good damage is in a fully buffed 40 man and a minore shooting attack does not make up for the difference when in a 10 man screen