Hijack? Please, this is a thread about nothing. :-)
Good used and NOS (New, Old Stock) tubes can be very expensive, particularly certain 12AX7, 6L6, 12AT7, 2A3, 6V6GT, EL34 if they come from RCA, Telefunken, Raytheon, Mullard, GE, Sylvania, Philips. A Telefunken 12AX7 will usually pull in $40 in nice shape and people will pay $150 for an old European (e.g. Dutch, English) or American EL34. If these are new in their boxes, they'll easily go $50+ and $200+ each. Most pre-amps have four 12AX7 and a power amp typically two 6L6 or EL34 per channel...
Thing is even good solid state transistors can be expensive, too. It's not just old tubes. A set of NOS Lambda 15K100/16K100 or Motorola MJ11015/MJ11016 BJTs can be just as expensive. These are 15+ years out of production and there is just as little call for replacements they might as well be tubes. Ironically some of the best places to find good heavy duty transistors is blower motor controls in old cars and trucks. Everything now is FETs, but back in the 1970 and 1980s more than a few higher end cars used BJTs (with HUGE heatsinks!) to continuously control motor speeds.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche