Many solutions exist in Paris. The cheapest ones remain the lockers at the main train station like gare du Nord, Gare Montparnasse, Gare de Lyon, Gare St Lazare.
Gare du Nord: very convenient when you arrive from the airport, the Eurostar or Thalys
See link: Lockers
They are located at the level -1 in front of the rail n°3 next to the exit called "cour des taxis".
Opening hours: from 6.15 am to 11.15pm
Price: small: 5.50€ / medium: 7.50€ / big: 9.50€
Citylocker: more expensive but at walking distance from our place
See link: http://www.citylocker.fr/site/files/index.php?lock_page=faq
The most convenient one is located at 82 RUE DES GRAVILLIERS 75003 PARIS METRO ARTS ET METIERS
It is 300 m from our place. You can book in advance your locker on their website.
Opening hours: from 8 am to 10 pm
Locker Size: 61 x 85 x 48 cm
Weight: max 30 kg
Price: between 7 to 16€ per locker per day (price varies according to the seasonality; on average 12€)
Website of the Tourism office with all the information about lockers in Paris: