It's 48#....not #48. 8^).
The magnesium risers (Mag)...were introduced in the very early 1970's, when nearly all archery companies were moving toward metal risers....mostly for their target models. There were many reasons for that in regard to shootability, performance, etc. But anyhoo....Bear introduced the Bear Mag. Takdown in three riser sizes. A, B and C, that could be matched with limbs, 1, 2 and 3, to create various lengths for just about any archery application.
They are not as valuable to collectors as the wood risers, but they are great shooters. The built-in sight system is the Bear Premier Sight, that was introduced in the mid 1960's in their wood riser Tamerlane target bow...then also put in a few other bows for target and hunting.
The price is what someone is willing to pay, but will usually be about 60% of the price of a wood riser takedown, if it is in great condition. I've seen them go for as little as $200.00 and as high as $450.00 for a like new model., but average will be around $250.00 to $300.00'ish, with wide differences due to condition....and who's looking.