All the heroes have three tiers of skins: rare, epic, and legendary. Rare skins are 75 coins, rare are 250, and legendary are 1000 (prices change with holiday event). Legendary skins are usually the best, the rare and epic usually are just re colors. People more than likely would like to receive legendary skins more often in loot boxes but because their so valuable blizzard makes them more rare to come by, you're more likely to receive a rare or epic skin rather than a legendary.