"It's pretty much the cheapest option you have for a subchannel outside of a still or color bars".
Unless you're in a state/city that uses highway video surveillance. On some cable systems throughout Oregon, there's an "ODOT Channel" which shows the highway surveillance cameræ throughout the state (same ones they show during the traffic reports on the local "news" broadcasts), with the local NOAA audio in the background (usually it's KIG98, even at the opposite end of the state from Portland.) I think there are ATSC channels elsewhere in Oregon that use a similar, if not the same, format on their subchannel stream. Since it's all technically public-domain stuff, I don't see how anything could be cheaper to run than that, unless you're just running a black screen all the time.