at (Pokemon) level 40 or 45, only rare candy XLs can level the Pokemon (to 50). each XL candy costs 100 Pokemon candies. to go from level 40 to 50 it takes 496 XL candies and 250K stardust (this is also related to the last 10% or so of a Pokemons CP with regular candies and that taking double again to go from 90-100 as it did 0-90