As I've said on here many times before, SSK barrels usually hold their value better than barrels from any of the other custom makers, and factory barrels. Why they are such a good investment IMO, well worth the extra price of addmission and why I've owned so many of them.
The approximate $540 now price is right baswed on its configuration. When I resell barrels, including SSK, I go by their honest condition, number of rounds fired and how much use I got while I owned them to come up with a percentage price to ask for them. The most honest is to discount off your purchase price for those factors when you know it maybe, but appreciation is a fact of life so you can also used "replacement" cost today as a basis to determine a fair selling price. Just discount for those factors above, and set your prices below that figure since it was bought at a lower price. How far below depends on what the market will bear and what cartridge it is chambered for. Some of the JDJ's cartridge are very popular and their barrels are in more demand than those chambered for other cartridges, and they will command higher prices. The TC factory 45-70 barrels were/are very popular and will usually do anything their owners want them to do with that cartridge. So an SSK barrel with the same standard chamber doesn't offer as much "extra" for actual use as the SSK wildcat barrels do. It gets it's extra appeal by being a hand lapped premium barrel with a well respected name, and one that will hold its value much better than a factory barrel will.
I know I didn't offer an actual dollar value, but I think only you can set that. I can say that I sold all of my SSK barrels for more than I paid for them because I kept them in mint condition despite some of them being shot a lot for many, many years. Even so I still gave the buyers a very good deal when compared to their current new prices from SSK, and they were all tickled to get them for those prices.
HTH,
Larry