I believe the major cost driver between the two is the fact that, AFAIK, nobody in the US is making tubes anymore, so what you're looking at buying are NOS (new old stock) tubes. Since available supplies of these tubes continue to dwindle, the prices just get higher and higher over time.
The reason that people are willing to pay that premium is that these NOS tubes were made to much more stringent quality requirements than most current production. A lot of the tubes coming out of Russia and China are notorious for very large tube-to-tube variation in their various performance parameters. I've read that current production tubes often show ten times (!) as much tube-to-tube variability as NOS tubes. With that kind of variability, an amp could easily sound completely different just from changing from one tube to another of the same type & brand.
The other comment I often read about NOS tubes is that they sound "warmer" or "rounder" or "more musical" than current production. I interpret this as the audible manifestation of that much greater product consistency. That they would tend to sound much better is certainly not surprising!
However, that's not to say that there aren't good quality, consistent current production tubes available these days. There are good Russian and Chinese tubes out there, but they often involve post-production testing and sorting by quality, a process that adds cost (and, therefore, means a higher price) through both the testing process itself and the "fallout", i.e. sub-standard tubes that are scrapped based upon poor test results. For example, I understand that Groove Tubes sells a lot of these test-and-sort tubes, and they have a pretty good reputation.
Another current production tube with excellent quality and consistency is the JJ brand, produced in the Czech Republic. Many people (including several Fretters) swear by them. I recently retubed my Peavey Delta Blues with JJ's, and am very happy with it--noticeably smoother and clearer tone than I had with the stock Sovtek's (Russian tube). Eurotubes.com is, IMO, the place to buy JJ's. Their service is absolutely outstanding and they'll even tailor the characteristics of the tubes they send you to meet your specified personal tastes (e.g. more headroom vs. early break-up). The owner, Bob Pletka, is a professional guitarist and just a super guy. They sell full sets of Hot Rod DeVille tubes in several "flavors" in the $55 - $60 range.
Hope that helps, Trem....