There are several reports on the subject that describe a very wide range of possible cost outcomes. There are even reports describing costs as high as $800/ ton today BUT with a halving of the cost by 2030.
With the leading technology from Carbon Engineering , calculations have been made between the range of 94-232 $ USD/ton of carbon dioxide captured.
Techno-economic assessment of CO2 direct air capture plants
Mahdi Fasihi *, Olga Efimova, Christian Breyer
A Process for Capturing CO2 from the Atmosphere
David W. Keith, Geoffrey Holmes, David St. Angelo, Kenton Heidel