It gets hard at level 60+. Especially for Priests. At 60+ there aren't much quest and you mostly grind, with AOE parties. Thus you need many hieros. Those are pretty expensive. For priests it's easier because they just have to cast their 'blue ball' (will post a picture soon, GD slow patching), healing skill that heals the whole party. But this eats MP thus they need MP hieros.
If you're bad with money making in game / not able to and/or willing to spend RL money on the game then you won't last long after level 60.