Is it common for tubes to outweigh the cost of the amp or preamp?

WOW!

Doesn't it seem too good to be true?
Just purchase a lower end tube amp, replace the tubes with expensive NOS, and it magically is transformed?

If so, proceed with power cable, headphone cable and special fuse. Now your total purchase will be $2500. 

Nevermind there are actual units that are possibly nicer sounding for $1600, right out of the box.

It's your wallet....carry on

@tablejockey I was never under the impression that I was buying magic. I'm just looking for moderate to good improvements in sound. Will I get that? I don't know.

I have gotten big improvements in sound from speaker cables, turntable cartridge, and speaker isolation. Improvements from interconnects have been negligible for me.

Headphone cables do make a difference but not enough to justify the price for me. Power cables are something I'd only upgrade if I didn't know what else to upgrade and had excess money lying around. And I don't know anything about fuses. I've seen people do crazy things like hang a string of beads from the ceiling or place a cello in a room to improve room dynamics. Keyword "Crazy". And I'm against pricey digital cables because... digital is digital. If I was going to buy all that stuff to bring the total to $2500 then maybe that attempt at an insult would have hit home. Sorry to let you down.

Of course I will be disappointed if there is no improvement in sound but that's a gamble I'm willing to take.

you can't go wrong with vintage tubes from the US or from Europe; even 80's era East European tubes are far superior to what is coming from China and Russia, which is what you find typically in even high end gear. Maybe the best way to describe vintage tubes is to look upon it like cooking; an old family recipe or institutional fare. 

One way to save a few $$ is to buy ugly tubes and pairs and quads that were all manufactured the same but we're labeled for various companies.Sometimes a couple of new production tubes alongside of a couple of NOS sounds good too.