Well, I looked in the latest edition of Paul Erano's pen book, and the sterling silver and gold electroplate versions of the Sheaffer Nostalgia fountain pens (the examples were from ca. 1990) were priced at $550 each. From what Sheaffer afficianado Rick Horne (The Southern Scribe) says
here (with picture) the
vermeil (the one you have) is one of the hardest to find in the Nostalgia series. Actually, he says on that page, only the solid gold model is rarer. By the way, I am not sure how recently the vermeil model there was sold...It could be quite a bit out of date, and I am not sure of the current value of the vermeil.
There is a 14K Nostalgia fountain pen
here with a 14K nib that is currently being sold for $850 USD.
Oh re: the 14K nib business, according to this post on alt.collecting.pens-pencils
herethe 14K nibs were found on the first issue of the Nostalgia line.
I guess if you had to put a price range on the vermeil model, you might say it's between $550 and $850 USD, just based on the info above. Depends on the condition of the pen, etc. The engraving will probably lower the value in this case, as the VintagePens.com.au page implies....
Hope this helps. In any case, it's a lovely pen