Maybe the extra cost is because the manual cover is so much more reliable?
Seriously though, it doesn't make sense, but not in the way you would think. At $1,800, the cost for the manual cover is not too far out of line with what third party covers cost, with the range being roughly $1,000 to $1,500.
What's really surprising then, at least to me, is that the powered tonneau cover is roughly the same cost as higher end third party manual covers. More surprising is that Rivian used to charge only $500 for the PTC.
After careful considerations of all the options, we purchased and installed the Lomax Tonneau caver by Agricover of North Dakota. We have had this tri-fold tonneau cover on Opal (our R1T) since March of this year. We have found it to be very well made with a well designed and produced locking mechanism. It is easy to use and very nice looking. It is made of quality materials and is water-tight. The cost was around $1,100 delivered. It was well packed and arrived in perfect condition.
In contrast, the Rivian OEM tonneau offerings are poorly designed and not very well made. Also, they are not very water-tight. In retrospect, Rivian should have left the bed coverings to the aftermarketeers.