The prices of champions and skins on League of Legends remained static for a very long time. It still has not been terribly long since Riot applied an actual regular scheme to its champion pricing, and now it seems to be applying a similar change in prices to champion skins.
The price changes are a result of the difference in quality of skins. While older skins were priced fairly relative to each other based on their quality, Riot has made tremendous progress with their artistic and coding talents since those earlier skins. Consequently, there are a huge number of skins priced at 975 RP of extremely varying comparative levels of quality. Changing the prices of skins is intended to balance this more evenly.
There are a total of 79 skins whose riot point price will decrease, including some former legendary skins dropping from the 1820 price range down to 1350 now that new legendary skins of greater quality have been released.
17 skins will be increasing in price from 975 to 1350 RP. These skins are, by and large, skins that were either expected to be legendary before release or were simply a huge change to the champion’s aesthetic, so none of the skins getting an increase is particularly surprising.
The full list can be found at the source on the League of Legends web site. Be sure to grab the skins increasing in price before the changes go live on April 25.