In January 2023, a Seiko SKX013K2 changed hands on Chrono24 for approximately 590 USD. The watch's value has steadily increased since the manufacturer discontinued production. In 2018, a SKX013K2 in new condition would have set you back about 250 USD, but by January 2021, the same watch demanded an investment of roughly 515 USD. This trend is very likely to continue: demand remains high, while supply is shrinking, and there is no successor model in sight.
Alternatives to the SKX013K2
At present, there isn't really a viable alternative to the SKX013K2 in the Seiko catalog. The 5 Sports SRPD55 is the most similar model in terms of design, but the SRPD55 measures 42 mm across, making it more a successor to the SKX007 than the SKX013K2. Furthermore, the current model doesn't have a screw-down crown and is only water-resistant to 100 m (10 bar, 328 ft). You can buy a SRPD55 on Chrono24 for about 270 USD.
The Seiko SNE573 belongs to the Prospex collection and measures 38.5 mm in diameter and 46.5 mm lug to lug, making it the near successor to the SKX013K2. However, while the hands come from the SKX line, the crown is located at 3 o'clock and the design of the indices is different. The watch is also one of Seiko's solar-powered quartz caliber models. You can buy a SNE573 on Chrono24 for approximately 470 USD.