I have one in 358250 & 358156 (of course). There a lot of Lyman/Ideal oddities that seem like they should be especially valuable, but really don't seem to be. I think Ben summed it up really well.
Antique tool collectors want these, but they are actually pretty common. As Ben said, until Lyman sold the company in the early '70's they would make what every would fit into the blocks. $50 is a fair price. The expensive ones are for like a 22 hornet bullet and a 45/70 bullet. Ones in the same caliber are much more common.I realize it's only worth what the buyer will pay. Honestly, I would consider it equal to a single, value wise.