So, why is it that tube amphs cost more than solid state / modelers?

[tunghaichuan***]

I would think that the components made to carry high voltages (caps, resistors, etc) cost more to make as they have to be robust.

True. Capacitors have to be able to withstand high voltages and this makes them physically larger than those with similar values but lower voltage ratings. 1/4W or even 1/8W resistors are common in solid state amps, where you need at least 1/2W rated parts in tube amps, sometimes 1W or 2W in certain locations in the circuit. Usually, the larger the wattage rating, the larger resistor.