It seems the larger the surface plate the harder they are to sell, and they tend to go for less money than the smaller ones. The big drawback is they are so heavy no one is willing to pay shipping. They are almost always sold to local customers that have their own way to transport them.
A few years ago, I purchased a 24" x 36" Challenge cast iron surface plate for $100.00. It had been for sale for several weeks, but no one in the vicinity was interested. Weighing in at 475 lbs. the shipping would have cost more than the asking price of $500.00. To put things in perspective the plate I purchased originally sold for $321.00 in 1964. In today's money that's a little over $3,100.00. I tried to find what they actually sell for today, but it appears they no longer offer cast iron plates. Everything offered today is Granite.
Don't be fooled by "asking prices" on eBay. Do an "advanced search" and check the box for "sold items". Often times you'll find larger heavier items go for considerably less than the initial asking price. That's how I arrived at the price I offered the seller for the surface plate I purchased. Several of the same size listed-on eBay had an initial asking prices in the $800.00 to $900.00 range. They ended up selling for $100.00 to $300.00 depending on the manufacturer and condition.