Vanderkindere, from Belgium, sells on 27 March 2018 an "attributed to Willem van Herp" Triuph of Neptune and Amphitrite, estimated at 20,000 to 30,000 Euro.
Stuker, from Switzerland, sold in 2014 (in the sale of a quite impressive private collection) an "anonymous 17th century Flemish" version of the work, estimated at 8,000 to 12,000 Swiss Francs. It eventually sold for 22,000 Swiss Francs. This version has since been attributed to Pauwels Casteels (active 1649-1677) at the RKD. Dimensions and material (oil on copper) are identical, the main difference are the additional putti flying in the centre of the image.
Gros & Delettrez, from France, sold another (much dirtier) version, again with the same dimensions and material, for 17,000 Euro in 2006. It was attributed to Pauwels Casteels. This may have been the same work as the Stuker version (I can't find any differences actually, which is otherwise exceptional), in which case it has had a remarkable cleaning, drop in price and loss of attribution.
Museum of Fine Arts in Houston has another copy of the work, where the artist is not named but described as "Flemish". This one again has some minor differences compared to the other examples, like the additional boy or putto in the water at the bottom centre.
The work for sale doesn't seem to be by Van Herp, who was more of a proto-classicist (and a good copier of work by e.g. Rubens), and is more in the style of someone closer to the Franckens. Pauwel Casteels may be the right name for this. The estimate for this work seems reasonable, perhaps a bit high; it is attractive, but shows some lack of brilliance when seen up close.
UPDATE: sold for 22,000 Euro, so the estimate was spot on.