I have a few stock answers at the ready, for that inevitable moment when I'm asked to evaluate someone's bicycle. "It's probably worth more to you than anyone else," or "I'll give you $10 right now," are my favourite shortcuts to the same point.
TL/DR: Your old Sekine is not worth more than $200, unless it's really nice, properly tuned, and has new parts.
There's an adage that an item is only worth as much as a buyer is willing to pay; and that amount depends a lot on both the item and the buyer. If I already have three bikes and my shed is full, I wouldn't pay much for another one, no matter how nice. On the other hand, if the bike isn't worth much but I really need one, I'd pay more.
Among the many thousands of Sekine bikes out there, probably only a few dozen are actually collector's items. These are the unique, custom-built, high-end racer models, preferably in mint shape. From there, the market interest declines, until eventually you're looking at a trailer full of miscellaneous rusty parts.
Vintage bicycle pricing is a fickle thing. People seem to buy them because they either have a sentimental attachment (collection), or because they just need something to ride (functional). The price depends heavily on the combination of functional and collectible, as every prospective buyer sits somewhere in the range. There's also the Hipster factor, which tends to inflate the value of some "vintage" goods, but that's another topic.
Eventually functional value reaches a maximum, probably around the point it's feasible to buy a brand-new equivalent. Collectible value, however, is only limited by an item's perceived value- the reason one painting will sell for millions and another won't. See chart. The line curves because most bicycles are just functional, while a few rare ones are worth collecting. Midway through the curve is the sweet zone of a functional bike that comes with a story.
Your bike is worth more if:
It has perfect paint. It's unused. It has forged dropouts. It has high-end parts. It's rare.
Your bike is worth less if:
It's rusty or dented. It's heavily used. It has stamped dropouts. It has cheap steel parts. It's common.
Just keep in mind that it's not a retirement fund, it's a bicycle. That Sekine is a machine built to be used, and as such may be worth more to you than anyone else.