Mar 31, 2022 · Other parts of the system include dry wells, sump pumps, downspout connections, window well drainage, and foundation drains. Type. Cost*. French. $1,000 – $10,000 per 100 linear feet. Trench / Channel. $3,000 – $9,000 per 100 linear feet. Underground Downspouts. $200 – $2,000 per downspout.. The average cost to install a new septic system will run you about $7125.00 for a complete septic tank system, including a tank, drainage pipes and drain field. Small, simple set-ups with no installation challenges could run as little at $4200 installed while systems for a large home with unique landscaping considerations could exceed $14000.Prices vary by size,.