Actually, I have no idea how much rope costs, but it was the first thing that popped into my head (I also have no idea if it's up to code):

Farmhouse Staircase · More Info

Sadly, stairs are expensive. The codes they have to follow are extreme and the builder who works on them has to be well trained and heavily insured. All of that "human knowledge + safety" stuff costs money. How do they "make it up"...they add costs to materials.

You can look at low cost lumber (the cheapest lumber available in the cheapest finish without any pretty bevels or round edges or anything nice like that). Then paint it. That's about where you are. And even that is going to feel like it is expensive. You could look at the cheap 70's metal rails that were covered in rubber. Those might save you $. They are ugly as sin...but everyone one and their dog had them in the 70's because they were cheap and quick to install. I don't even know if they are to code any more...but they are out there. You could also check Habitat for Humanity Re-Stores to see if they have any materials that could be used to build the banister....could save $...or could be a complete waste of time. It depends on their stock that week.

Sorry but stairs are expensive. Pretty is expensive. Ugly is cheap. Good luck.