The cause leads to the United States, because collectors here can finally import the second-generation GT-R legally now that 25 years has elapsed. As a classic car rep says in the CPC article, though, the trend only applies to "really clean examples," ones with low miles. Road & Track spoke to a couple of companies importing them into States now, and they report that prices have tripled in some cases, and special editions like the R32 GT-R Nismo have gone beyond that.
If you're not looking for unicorns or Newfoundland Ponies, however, the folks in the business say you can find a reasonably priced examples. Because they were performance cars popular with the modding crowd, akin to our last-gen Toyota Supra and Mazda RX-7, there's a wide range of wear and tear. The inventory list for importer Montu Motors shows a couple of unsold GT-Rs for mid-twenties money. Chris Bishop at Japanese Classics thinks the present spike is down to early adopters; once they skim the cream and more model years can be imported, "prices will level off, and then go down."