Maybe look at where the top sites that you want to see are, and pick based on that? If your time is limited, you might want to be close to your tops sights if they are near the Ile. If your schedule is more leisurely, you might be more inclined to save a little and stay farther out. I've only got one full day in Paris, and originally wanted to stay on Ile St Louis because that is where all the sights I want to see are concentrated, but the affordable options are already booked for when I want to go. So, I am planning to stay in Saint Germain de Pres area, still very close, and while definitely not cheap, there are some less expensive hotels. I booked a room at Hotel Saint Paul Rive Gauche (now, hopefully I won't find out any horror stories from anyone about their stay there!).