<b>Are these pritty blue tube amps hard to build and expencive ..?</b>
Yes and yes.
First, the tubes you like to look of so much are not signal path tubes, but 866A mercury vapour rectifiers. Basically they're diodes but these require extra circuitry to pre-warm them before the HT is applied or they burn out. They're not very expensive when you can get them, but they can be noisy, and so you might have to get a few before you get two quiet ones. To my knowledge they're not made any more.
The large tubes in the centre are the power tubes, in this case 845s. Chris has the HT on these at 1080V, so unless you are experienced in working with voltages this high, tackle something else first. The circuitry in them is quite simple looking, but it's implementation requires a bit of detail in construction and attention to layout etc, both for optimum sonics, and most important for safety. 1100V can kill you real dead, real fast. With experience and the correct approach and tools, you <i>could</i> build one.
As for cost, it depends on your budget, and how high you want the performance to be. The iron ie output and power transformers and PSU chokes could add up to a considerable sum of money. Some like the chokes and power trans, you might find as surplus or NOS, but the outputs will probably cost, depending on brand and performance level. Caps and resistors will also need to be high quality and rated for service at these voltages. Your minimum if you found lots of surplus would be $US1000 at least.
I have an 866A in my tubebox, and just picked up a transformer cheap that I'll use with it to make a nice blue 'nightlight'.
If you just want something that glows, try an OC3 / OB3 / OD3 gas discharge regulator, maybe $US5. There are other signal valves that glow too, but few anything near the 866. The tube lighting up and looking good is a side benefit. If you want a tube amp for the sonics, I can help you with ideas if I know your goals, budget and experience.
Cheers
This is a pic of one of the tubes I'll build into some amps next year. (pic from Bas Hornemans' site)