I can't save give you a lot of details, but that blade is available at Messicks.com for around $30.30 each. Add shipping and you are back to your original price or more. I didn't find your RFM's model on Bush Hog's site, but the info I find on your blade is that it is A 3/4" arbor and 21" long. The nut and threads are left hand. The blade shown is also a CCW cut. The model of your cutter seems to be from 2001. Bush Hog may have OEM'd that cutter from Woods and put their labels on that. It's common practice. Later, they could have developed their own series and dropped the Woods RFM from their line. That's all supposition, but you might find your part under Woods brand RFMs, but a model number off of it would be good. Was there no decals on the cutter except for "Bush Hog" decals?
Good luck. I may be just repeating info you've already found, but it sure looks like your blades are more expensive than most, probably because they are very rare.