It truly depends on what stair lift you are looking for.
Straight stair lifts are usually the cheaper one of the group averaging around $4,000, that includes the product itself, installation, and service warranty.
Curved stair lifts usually average around $11,000-$15,000, price varies depending on length of staircase, brands, and features.
Outdoor stair lifts can average from $5,000 all the way up to $12,000 depending on type of staircase and features of the product.