Almost every Jefferson nickel is worth just that -- one nickel. All Jeffersons in worn condition are worth a nickel. If you find one in uncirculated condition, or nearly so, it will be worth more than a nickel, but not too much more.
If you find a collector who enjoys Jefferson nickels, and there aren't many, Jeff could probably sell his coin for a few dollars.
Look at the superb Jefferson nickel in the picture. It is the most valuable date and mint mark (1939D) and it is in wonderful shape, assigned the lofty grade of mint state MS63 by Numismatic Guaranty Company, a highly respected coin grading company. The retail value of this coin is less than $50 US dollars. Another good date, 1950D, is worth about $15 when in MS63 condition. All other Jeffersons are worth very small amounts.
Also, as you may know, Jeff, the US Mint issued Jefferson nickels with a silver composition during World War II, from 1942 to 1945. These so-called War Nickels are worth a tad more than regular Jeffs and, at a high silver price, their 0.0563 troy ounces of silver starts to kick in to coin value. A 1944P war nickel can reach $10 value, but only in fully uncirculated (MS63) condition. The one in our picture shows the P mint mark on the reverse (common to War Nickels). Other mint marks are D and S, with values roughly the same. War nickels always have their P, D or S mint marks prominently displayed on the back, over Monticello.
In summary, Jefferson nickels are not worth very much unless they are in superb condition. Even then, almost all dates struggle to get past $5 catalog value.
To answer his specific question, the best I can do for Jeff's proof nickel in a PCI slab is look it up in the Grey Sheet, a conservative price guide for US coins. It says $8 (US dollars) in PR64 and $10 in PR65. As you probably know, the PCI slab is suspect. If the coin really grades PR67 deep cameo, it is worth much more than $10, but I would be hard pressed to say how much more. The main source of value would be the deep cameo, which is commonplace on modern proofs, but not so on coins dated 1955. Venturing a guess, if your coin really does display deep cameo and is better than MS65, I'd say $50. But that's only a guess. I have seen PCI slabs marked Deep Cameo with essentially no cameo showing on the coin, let alone deep cameo.