I need to buy...
Decathlon
You forgot to pack a coat, or the one you brought is too light (or too heavy). You didn't bring comfortable shoes, or you walked so much that your sneakers broke. It's been raining for three days and you didn't bring an umbrella... These are all things that happen in Paris, but you don't have to spend a fortune. In front of the Madeleine church (Metro Madeleine- near the Place de la Concorde and the Paris Opera) is Decathlon. A store of sportswear, shoes, and accessories of quite good quality and at a low price. I guarantee that whatever your problem is, you can solve it for less than 20€.
Macarons & Local Goodies
Macarons were invented at a patisserie called La Durée. You can find it all over Paris (and even at the airport), but you can still go to the historic location at 14 Rue de Castiglione between the Place Vendôme and the Jardin des Tuileries. A box of 6 costs €17.10. The price of elegance!
Since you're in the area, enjoy a delicious tea time at the old Angelina patisserie at 226 Rue de Rivoli. It's open every day from 9:00-19:00. It's an institution!
**If after filling your belly with these delicacies (as delicious as they are expensive) your wallet is empty and you still haven't bought a box of macarons for your friends at home, don't worry! **
There is a very good, very nice, and very cheap alternative. La Maison Georges Larnicol has several locations in Paris. They specialize in French sweets and chocolate, and they make very delicious macarons for only €1 each! With less than €10, you can buy a box that will last up to 5 days in the fridge.
You can buy them at:
7 Rue de Steinkerque (Metro Anvers)
14 Rue de Rivoli (Metro Saint-Paul)
19 Rue de la Harpe (Metro Saint-Michel)
Don't leave without trying one of their salty caramel Kouignettes! My mouth is watering just thinking about them...
Souvenirs
Paris is expensive, we know. So wherever you buy them, you're going to pay for them with blood, sweat, and tears. The good news is that there are three small streets in Paris where you'll find so many stores next to each other that, by mere competition, they'll give you more competitive prices.
In the Latin Quarter: Rue de la Huchette (Metro Saint-Michel)
In Montmartre: Rue de Steinkerque (Metro Anvers)
Louvre area: 97-242 Rue de Rivoli (Metros Tuileries, Louvre-Rivoli, Palais Royal)