Depends on what someone is willing to pay. I used to own the fullflex nymph and midge. I liked them both but always regretted letting go of the nymph.

Agreed.

I missed out on one recently on eBay, max bid was $165 I put in last minute, sold for $167.50 kinda mad I missed out.

I think the 175-275 range.
I go that high simply because there were only 2 5wt models made if I remember and because of that they are more rare. While rare doesn't mean $ in all cases, it does here because demand outweighs supply on these no longer made rods. At one time I had 5 of the Orvis yellow phillipsons made rods, the nymph IMO was the best of the bunch.

Edit:
I stand corrected. There were 5 made. However in all my years I have only ever seen the midge or nymph ever come up for sale. i still hold firm to my original price range though. That time frame did not see many 5wt fiberglass fly rods that Iam aware of, at least those marked as 5wts.
Then again I have a terrible memory so maybe Iam wrong. Either way the nymph, to me, is worth every penny at 250

Another aspect of that rod (and all other Orvises at that time and it may still be true) is that Orvis invariably rated them a line weight heavier than they really are. I'll bet a 4 wt works well on it.