That was the savings estimates at the time from sources in the government iirc so I think they would have included the relative durability of the cotton bank note in the calculations. [0] Of course that's just the savings not the costs but a lot of those are one time as different coin op mechanisms get updated to accept dollar coins.

[0] The $1 notes also go through a lot more use in their life so they degrade much quicker iirc and a bit over 40% of the bills in circulation are $1 bills too so switching over to a coin that lasts decades instead of 6-7 years (https://www.federalreserve.gov/faqs/how-long-is-the-life-spa...) would really cut down on printing costs.