It'll always be more expensive to extract carbon from the atmosphere, but if it produces a valuable byproduct and the power to run the process is very cheap (excess solar) it might still pencil out.
When you have surplus photovoltaic capacity — an increasingly common situation — you could turn it to electrolysis. You still have to pay for the equipment, but it sounds like it could be pretty cheap.
It would cut out the cost of mining, and possibly a lot of transportation costs, if you can use the products nearby.
