There's only 2 successfully completed eBay auctions (historical data) in the past few months. They are not extremely accurate because they were not in new condition, but one sold for $15 with monitor (working unit but it did look like it needing a good cleaning), and a clean but untested unit without monitor (and missing the expansion cover) went for $22 (the shipping was considerably cheaper since there was no monitor included, which does affect the max bid).
The machine itself is key here, because the CM-5 was Tandy's low-end color monitor and they are quite easy to find. As a potential buyer, my concern is the added shipping from the CM-5, but I am still willing to bid/buy it as a whole if that makes it easier to manage (which is completely understandable...I'm sure you've got plenty of other things to go through and list for sale from the estate). You can always list both items within a few minutes of each other, but on separate auctions... Usually, when a seller splits items like this, the high bidder for the main unit will have a higher interest in winning the remaining pieces, especially if the winning bid was lower than what they were ultimately willing to pay. A lot of eBay has to do with the emotional factors.