That's a nice mold for a good sized storage ingot. I'd be afraid, though, that an ingot that size would freeze the melt or at least drop the temp out of the casting range if added cold, and might be awkward to handle easily if preheated. For casting out of a 20# pot I'd like to stay smaller (maybe for a Magma 40# pot, that'd be just the ticket).
And while I'm not as cheap as during my salad days. I still think $50 is a lot for a mold, especially since it's just something to hold and cool the lead into a convenient shape.