here's my references before you ask for a source
http://na.leagueoflegends.com/en/content/skin-rp-price-changes
http://leagueoflegends.wikia.com/wiki/Champion_skin/Retired_skins
http://leagueoflegends.wikia.com/wiki/Champion_skin/Available_skins
skins originally priced at 520 that changed models, before a character was reworked: 26
this means you had 26 skins out of everything that changed a model and weren't 975. i would even say 25, but waterloo miss fortune changes tiny bits
adding on skins that changed models and were lowered before the introduction of 1350 and 750: 36
wow holy hell such a wide range of model changing 520 skins. but wait, now we add another 36 that were added in the 2013 price reduction. now half of the model changing 520 skins are because of riot reducing skin prices because of the 1350 tag. such a terrible, terrible thing they did.
we have 109 model changing 750 skins. 40 of them are skins that used to be 975. ALL of the skins in the brand spankin new 750 tag (when it was introduced, obviously) were 975. and only changed models. pretty much all pre-2013 975 skins that are currently 975 only change the models. there's a few that change effects (oktoberfest gragas, lulu's release skins, muay thai lee sin to name a few) but it's not anywhere near the same quality we have now
skins released in 2016: 52, 38 of which are 1350, 5 are ultimate / legendary, the rest are 975 / 750, meaning 82%~ of skins were 1350 or above. sucks ass.
however, in 2015 87 skins were released, 32 of which were 1350 or above, meaning only 37%~ were 1350 or above.
i didn't bother counting every skin for 2014, but it looks more like 2014 was even between >975 and <=975
so yeah having one year for mostly expensive skins while the past 2 - 3 years have been mostly cheaper 975 / 750 sounds fair to me, and having the 1350 tier introduced and used more for better quality is pretty fair for having model changes shifted to being cheaper
don't forget how model changes are also much higher quality AND cheaper than before