A League skin has three kinds of “worth”: what it costs in RP, what it's worth in Orange Essence if it's a shard, and how rare it is. What it doesn't have is a resale price: skins can't be traded or sold, and selling accounts breaks Riot's terms.
1. The price in RP
The store price, which the gem roughly tracks: 520, 750, 975, 1,350 RP (Epic), 1,820 RP (Legendary), 3,250 RP (Ultimate), with Mythic, Exalted and Transcendent skins sold through special currencies or events. It's the easiest number, but it says nothing about how easy the skin is to get today.
2. The value in Orange Essence
As a shard, a skin has a fixed Orange Essence value. A 1,350 RP skin, for example, costs 1,050 OE to unlock and disenchants for 270 OE (or 675 as a permanent). The full table is in reroll, disenchant or unlock (values from the League wiki).
3. How rare it is
This is what makes a skin special. A skin that's always in the store is Common, however expensive. A Legacy skin is Uncommon. Skins sold only for Mythic Essence or through events are Rare, and skins that were only ever handed out, like PAX codes, collector's editions or ranked rewards, are Unobtainable. How to tell how rare your skin is.
Look up any skin
Search your skin on lolskins.io: its page shows the price, the rarity level with the reasons, the skin shard values and whether it can drop from loot. And if you just want to see the most valuable things in the game, here are the rarest skins in League.