Fauve Paris sells on 17 June 2017 an "Italian School, late 16th century" drawing of Christ and the Samaritan woman, quite large (37 by 50 cm) and estimated at 400 to 600 Euro.
National Trust museum / house.
The drawing is certainly better than e.g. the Welsh copy, and is not identical to the original either, as it has a landscape orientation instead of a portrait one. As far as I can discern it, it even looks better than the supposed original painting from Siena, which makes me wonder whether it could be a final preparatory drawing by Venusti, instead of a copy by someone else after the fact.
I can't find any comparable drawings, so valuing this is hard. It has some damage, so as an anonymous copy the estimate seems right: if it is by Venusti though, the value should be at least 2,000 to 3,000 Euro.
British Museum. This explains the better quality of the drawing, and removes the link to Venusti. Thanks for the update!