Exterior French drains cost approximately $10 to $30 per linear foot, based on the depth and width of the drain, according to Cost Helper. A 50 foot drain may cost anywhere from $500 to $1500. Other factors affecting price include geographical location and ground conditions.
Interior French drains, those placed in basements, cost a bit more. They average in price from $6,000 to $10,000, as reported by Angie's List. The price is based on the basement's perimeter measurements since the drain is normally placed around the perimeter of the basement. If a sump pump is installed, the cost may be higher.