I haven't had time to digest all of the good info y'all are providing this morning, but I have tinkered with the furnace some.
I started by sanding the oxidation off of the heating element leads and rechecking them. It turns out that only one is bad. The other three all have a resistance of 7.8 - 7.9 ohms. The bad one is melted in two where the lead comes thru the firebrick at back of the furnace. I'm going to pull it out and see if it's possible to repair it. I've read that it's possible to solder this type of wire with special flux and solder. I'm asssuming this is Kanthal wire.
The reason why the replacement elements are so expensive is that they are embedded in a thin refractory brick. So they are not directly exposed to the interior of the heating chamber. If replacement is necessary, I think I will try using an inexpensive "regular" element. I would just need to find a way to support it.