This is a work by listed South Carolina artist Randall McKissick (born 1946). Based on comparables of his works sold at auction, the value is $5000.
The subject certainly helps its sale-ability, however the subject of Georgia O'Keeffe as depicted by another artist does not make this more valuable. The collector market, in terms of high value, would only primarily pertain to works directly made by O'Keeffe herself then followed by photos of her taken by famous photographers and such.
Just checking in to see if you viewed your answer and if you are seeking further assistance?
Please let me know.