The idea that a COA or signing an open edition print adds much or any value is a misconception, IMHO. You may get away with adding a few bucks to cover the cost of the cert and producing it, but it adds little or nothing to the value of the print. We are selling decorator art, and to imply there is some collector value by signing it or supplying a COA is, IMHO, disingenuous. It is a nice-to-have kind of thing, but it adds no real value.
Serious collectors do not put much, if any, value on open edition prints simply because they are mass-produced with an unlimited supply, and the signature will mean little or nothing. On rare occasions, they may add value after the artist dies because they can no longer pump out more copies. But that artist will have to have reached some level of fame.
I am a dealer for most publishing houses representing the best artists working in the Signed and Numbered Limited Edition business. I have been doing this as an artist, publisher, wholesale distributor, and retail for over 50 years.
Some if not most of the top, the most popular artists as only do smaller sizes of and for the open edition prints they are only getting $5-$15 more if it is signed.