you will find a better price from a carpet dealer who will also do the installation. I have a similar situation, 2 square landings and 3 groups of steps and a hall. I had it redone aout 12 to 14 years ago and the whole deal was under $1000...and a very high quality carpet. But wondering why you want tread showing on each side of each step? maybe that is upping your costs? And you are simply opening yourself to every day dusting. Somehow the dust on stairs shows up really well. I remember my mother running the dust mop on the stairs every day...so when It came time for me to carpet stairs, I didn't leave any wood showing.

Just to be sure of what you want - Your stairs are carpeted now, and you want more carpet to put over them? That could be a hazard for someone not used to walking on them, with the increased thickness.They would have to be attached with carpet tacks just like the ones you have now. You can rent a rug cleaner or have someone come to your house to clean rugs, which may be less expensive and will be safer than a heavy runner on top of existing carpet on stairs.

Do you mean carpet runner on top of carpeting... Article/story on houzz offers this: "Also, take note of the recessed step lights along the wall. We started this ideabook talking about safety. Good lighting is critical, and lighting focused on the treads allows you to illuminate the stairs safely without overlighting the rest of the space. The runner pictured here also protects the stairs and makes them less slippery, reducing falls." https://www.houzz.com/photos/second-staircase-traditional-staircase-seattle-phvw-vp~427162

Your designer will have to the trade resources for all kinds of high quality things that you cannot buy retail. Designers are not shopping services though. Even with a design package like Designer by Your Side, service minimums, and specific design processes, will apply.