Throwing money at a wine purchase is an easy way to find excellent wine. The real art in selecting wines will be to find a very acceptable bottle for 15€ to 20€; a challenge but not an impossibility.

If you are interested in Bordeaux wines, I would try them at as many price points as I could to really understand the quality range available. To start, Carrefour or Monoprix do have a number of interesting bottles at this price. If I were to spend 100€ for a bottle, I would be very interested in its provenance and I might avoid grocery store options. However, larger Carrefours have their top wines stored in specially designed, temperature controlled, wine cellars. In Paris, the Carrefour at Porte d´Auteuil has such a setup in their wine department which itself is sizable. They also have people on hand who can help you with a selection but do not expect them necessarily to speak English.

At under 20€, provenance may not be all that critical. You could simply go to the nearest Monoprix and pickup an interesting Bordeaux but you could also go to one of these cavistes who might be able to help you with a more informed selection, some offer wine tastings:

• Lavinia
  3 boulevard de la Madeleine, 75001 Paris
  A well known caviste with an extensive selection. You could spend a good amount of time here just looking.
  
• Les Caprices de l’Instant
  12 rue Jacques-Cœur, 75004 Paris
  Raphaël Jimenez is a wine connaisseur who can guide you towards the best wines available at your price point.
  
• Le Quinze-Vins
  1, rue Dante, 75005 Paris
  Hugues Rondouin was awarded 2013s caviste of the year. This would be an excellent location to spend some time and learn about wines.
  
• La cave Philovino
  33 rue Claude Bernard, 75005 Paris
  
• La Dernière goutte
  6, rue Bourbon-le-Château, 75006 Paris
  A small location specializing in wines from Bordeaux and Burgundy. Wine tastings and visiting wine producers come each Saturday to talk about wine and their products.