Took a look a few months ago but ewong is busy with other tasks so didn't do more.
For Windows probably around 500 Euros a year for a basic certificate.
An EV certificate costs more but needs a hardware token. Not sure if this can
be even used in automation.
Apple is becoming a sh*tshow. Very proprietary signing mechanism and you need
to upload the binary for notarization. Cost is $100 a year but I didn't look
much into it. You need a real mac to do this too which adds to the cost.
As a sidenote. I expect them to lock down macOS eventually so in the long term
it becomes a giant smartphone like system with only an app store. Switch to
arm was only the start. Yes I know this was done to free their valued users
from the evil clutches of Intel. If you believe it I also have some very high
value swamp area for sale. Please inquire.
There is a bug open for adding this but one step as a time.