My husband and I stayed at Philippe's place during our visit to Paris and found it charming and perfectly located (right next to the Jardin des Plantes, Notre Dame, etc.) We were able to walk everywhere quite easily. The neighborhood is also quite nice, with lots of cute bars, cafes, markets, and parks nearby.
Philippe and his family are lovely hosts, and had us over for wine one evening. They brought wonderful pastries each morning, and were quick to answer any questions we had.
If you're looking for an affordable, well-located place in the heart of Paris, stay with Philippe!