Just some tips to help you remember the price of skins by their quality. (This does not apply to rather old skins such as Gentleman Cho'Gath or Ice Toboggan Corki).
390/520: Basically just changes to coloring or minor textures, no longer being made iirc
750: Generally is an update to texture on the champion with a few quality of life changes to skills if necessary (Order of the Lotus Irelia's Transcendent Blades, SSW Talon's Rake and Shadow Assault).
975: Update to textures on the champion as well as abilities, new recall animations.
1350: Update to champion and ability textures, new recall animations, and new sound effects.
1820: Almost like a minor champion redesign, updates to champion and ability textures, new recall animation, new sound effects, new jokes, taunts, etc.
3250: Updates to textures for champion and abilities, new recall animation, new sound effects, new jokes, taunts, etc., new summoner icons, banners, and evolutionary stages.