How much was that Sekine bicycle worth back in the day when it was new? I've managed to find a few retail prices scattered in old newspapers. Here's a listing from a Winnipeg bike shop in 1975:
SHS 270 - $250 - Deluxe cro-mo lightweight ten-speed
SHL 271 - $169 - Mixte hi-tensile lightweight ten-speed
SHC 270 - $160 - Hi-tensile lightweight ten-speed
SIA 271 - $130 - Hi-tensile
SIA 101 - $124 - Hi-tensile
Listings from previous and following years show that over time, prices climb (predictably) ever higher.
Now that we know how much they sold for back then, we can approximate what a Sekine would cost today using The Inflation Calculator. Of course, nowadays a lugged steel frame would probably cost a lot more, but you'll likely be getting a better product.
If you're buying a used bike, always subtract for wear, keep in mind the components you might have to replace, and remember that vintage does not necessarily mean good.