Those Xp orbs sell on convenience. It does make sense that its more expensive, because it takes 0 work. Daily resets, you need to do 10 dailies each time yourself. TOSH farm, easier, but takes hours. XP orbs, you click them and boom, done. Its instant, with no work done.
But honestly, ANY powerleveling is strongly not recommanded for new players, unless you HATE questing, which is like... 70% of the games lol, and is like that for most MMO's. When you powerlevel, you pay gold/$$ to skip gameplay. Powerleveling is good for your second/third/fourth class, but never for your main class.