I would just install a new carpet. You are saying runner, and its not a lot different. Get a pretty carpet, and one that doesnt show every single mark it gets on it. I notice the one you have is very dirty looking. The way your stairs are built, new carpet seems the way to go, unless you want to spend a whole lot of money.

There isn't going to be a "cheap" way OTHER than have a wood guy make FILL IN treads . Then paint all the treads a soft black, the risers white and then do a runner that is two inches wider than what is there. The stairs are not a straight run.....any runner will need to be customized to the stair.

Or......call Home Depot and get the inset runner replaced : ) installed,

I agree install the new carpet just like the old one and done or be preapred for a $$$$ job no cheap way out.

Installing a new runner would probably be best.