Directly heated triodes like the 300B will probably always be expensive because there really is no cheap way to get the sound people expect out of them. They live and die by the quality of their output transformers first and foremost, and good ones are hand-made in low volumes and can cost 4 figures just on their own. The weird catch-22 of DHT amps it that they are pretty simple and elegant in terms of circuit design (and naturally require no negative feedback), but they are very sensitive to the quality and composition of most of the parts that are directly in the signal path. They also require special care and feeding in terms of power supply design to prevent unacceptable levels of hum.
This, combined with the nature of the tubes themselves (low volume and still largely hand-made, thus expensive even in new production form) means that there is really no opportunity out there to swoop in and undercut existing products with a good sounding 300B design that costs significantly less.
They do sound lovely when done right though. Not just the 300B, pretty much any directly heated triode is great. It's not everybody's cup of tea though, and I would encourage you to seek out an opportunity to hear one for yourself sometime. Also, keep in mind that 300B amps in particular can actually vary quite a bit in presentation because a lot of designs use driver stage tubes that don't drive the 300B properly. Not overcoming the 300B's miller capacitance is the primary culprit, which is often the case with designs that use a 6SN7 driver. This is why some 300Bs are overtly "romantic" sounding and the tube has that reputation. When driven by a capable driver stage the 300B is not nearly as "tubey" sounding as it's reputation would have you believe.