The $36 calls are as you note either poplar or cedar, mostly poplar. Both box and paddle. They are bare wood (have no finish coat) and and lack the fined sanded finish of the call pictured which has mutliple, more expensive, woods and has weather resistant protective finsh. Hence, the additional $4 cost.
Should someone desire to purchase the $36 call of plain ufinished cedar ot poplar I would be more than happy to make them one.
What does "gauge" mean anyway? As used here gauge means the number of round lead balls the same diameter as the inside of the barrel (aka the bore) that it would take to weigh one pound. It takes 12 lead balls the same diameter as a 12 gauge barrel to weigh a pound. The smaller 20 gauge would require 20 balls of that barrel diameter. The larger bore 12 gauge would require 12 lead balls of the bore diameter to weigh one pound.
In general the number of pellets in a shotgun shell...