I wouldn't jump to panacur. There is no reason to think worms are the problem. Lets first assume now that your third snake is fine. What can cause two snakes to acutely have a terminal inflammed problem in the neck while under the care of a non-herp person (an assumption on my part). I've seen inexperienced people attempt neck grabs on snakes with the impression that it is the correct way to hold a snake without getting bitten. Other physical trauma like being caught in the lid might also be a possibility.

If my assumption is incorrect, then medical reasons are uncommon and vague since this is an uncommon presentation. A necropsy would have been your best bet to find out what went wrong.

I personally don't know why the purchase price of a snake determines the worth of a vet visit but that's a decision for you to make. In the mean time, I would measure it's weight and watch for fluctuations. Take photos and compare to see if it is increasing in diameter around the neck.

I hope this helps a bit,

Ian