This is going to depend a lot on what range of French door models you buy from. Not including installation costs, higher-end French doors can go for $3,000 or more since these designs will often feature fiberglass or aluminum cladding. Since these doors are typically made to order this will also drive the cost up. Stock French patio doors by contrast go for far less, and you can find examples for as little as $400, not including installation costs. These will often be premade examples made of steel or a lower grade wood material, but can still be quite durable.
DIY or Pro?
Because balancing double doors can be inherently technically challenging, installation is usually something that is best left to the professionals since a mistake could become rather expensive fast. Having said that, if you are an avid DIYer with a good amount of experience you can try your hand at French door installation. It certainly is not the most technically demanding job, but you will need to know some basics about door installation.
French Door Longevity
It is not unusual to see French doors with twenty-year warranties, so this gives you a pretty good idea of how long you can expect a French door to last. Quality French patio doors are typically quite robust in their construction and this allows them to withstand the elements for decades without faltering. When you recall that exterior French doors also boost curb appeal, this makes them an excellent investment. It is not unusual to inherit French patio doors from a previous homeowner when you buy a home.