It's all quite relative. Juniors usually go for 15-20€ph (I've heard even lower rates, but not my job to convince anyone to respect his own work), seniors can range between 60 and infinity. If another developer needs twice as much time as you, this means twice as much time waiting, and certainly a lower hr for him. It's just all too relative. How useful your app is, is your client in real need, is it something totally new and/or custom? Just too many parameters.