i'm a tube amp builder myself (i also do fx and repairs and mods
i've build 2 amps up till now.
The first one is a Mashall "18 watter" (1974x) lite2b, modded to my taste.
I took the tubes (2x mullard 12ax7, 2x mullard el84, 1x mullard ez81 all the tubes where in nos condition), chassis, sockets, and transformers from an old geloso PA tube amp from the fifties (i gutted it completly). I got that amp for free. The total price would be around 50€ as i've only had to pay for the resistors and capacitors and potmeters.
The second one is a Marshall 2204/1987 hybride. Again it's build inside an old geloso tube amp (so i reused it's chassis, tubes(2x mullard 12ax7, 1x mullard 12at7, 2x mullard el34 all the tubes where in nos condition), ...), but now from the 60ies. I got this amp also for free
. This amp was a bit more expensive to build because i needed more stuff (relays, switches, more pots, more resistors, ...) and i've installed a new transformer to power the relays. I'd guess the total price to 100€. The amp is constructed, but i'm still in it's modding fase.
The third amp is currently a design project. I've bought an old 100w guitar tube amp for 85€, and it only had a slight problem (slight hum and a bit of crackling). I thought i was going to be able to fix it, but it was build on a pcb and it was in real bad condition so i ripped it out. The amp was by the way a hiwatt DR103 copy that someone build in 1973. It contains a good PT and OT set, 4x marshall el34 and 4 sovtek 12ax7, a good reusable alu chassis and a nice tolexed wooden headcab!!.
I'm planning on making a soldano slo100 out of this.