I can't really understand why the cheaper retail price Chinese-made Celestion 12" loudspeakers, eg the Hot 100 should sound any "worse" than the more expensive speakers, as they cost virtually the same to make. They all use the same baskets, motor units, suspensions. I also suspect the use the same voice coils and pole pieces, and use either 35oz or 50oz magnets. They probably use different cones, but I suspect that these will be very similarly priced.
It does beg the question why there is such a big price difference between a G12T-100 and a G12M. I suspect it is not due to the coat of paint on the basket and the plastic cover........
The quality of the cone makes a huge difference to the sound, however as pointed out above, the price difference between a really good cone and a poor one is not that great, although many manufacturers still seem to want to always go for the cheapest option.
An area where cost can be cut is in the pole piece/ motor unit alignment. For high efficiency, the voice coil gap should be as small as possible, however this means that the pole piece/ motor units need to be machined (and assembled) to very high tolerances, and that the alignment of the voice coil needs to be very accurate. Both these criterion add significantly to the cost.
The argument in differences in quality of hi-fi and guitar speakers is spurious. The original G12 speaker was developed in the 50s as an efficient (amplifier power was low) loudspeaker for general use, the fact it turned out to sound great for guitar is purely a coincidence.
Speaker efficiency was important and can be increased by increasing the magnetic field strength in the voice coil gap; however there is a significant trade off with bandwidth, so hi-fi speakers are typically 10 -15 db (most are around 85 db at 1 W) less efficient that guitar (or indeed PA) speakers (95-100 db at 1 W). Additionally, very few hi-fi speakers have a 12" driver in them, and the cabinets they are closed-backed and non resident, so you aren't really comparing like with like.
Furthermore, you definitely don't want a tweeter in a guitar amp!