There is an extra polishing step which I’m sure adds something to their cost. But I also think it's mostly due to them lasting as long as they do. A roundwound string’s lifetime is measured in months. A flatwound’s is measured in years. Or with TIs it’s sometimes measured in decades. Kinda like eyeglass frames. They know most people are going to put off purchasing a new pair of glasses for as long as possible. So they get as much of your money as they can up front.
Truth is, flatwounds are going to be significantly cheaper on an annual basis compared to your average set of rounds, assuming you replace rounds multiple times per year. If you buy something like HiBeams ($25) and restring every three months you spent $100 compared to a set of TIs that will set you back $85 most places. So you saved yourself $15 over rounds in the first year alone. Year two the TIs will still be going strong and should easily last anywhere from five to ten years. So there’s zero replacement cost during that period.
Another way of looking at it is by dividing the cost into the number of months useful life before replacement.
TIs = $85 amortized over 60 months (5 yrs) is $1.42 per month.
HiBeams = $25 amortized over 4 months is $6.25 per month.
My current set of TI flats (4 string) have been on my G&L L-1000 since 2014, almost 10 years. The cost doesn’t bother me on a running basis.