At the AAM conference this week, the FAA made a statement that acknowledged that RID modules were hard to get (they didn't address cost). They said there will "probably be more announcements before the deadlines. All things are on the table." I read that as they may have to extend the deadline. Hopefully more manufacturers will step up with lower cost units and an extended deadline happens to allow for them to get to market.
In the meantime, here is the least expensive model I have found so far. You will need to add the antennas (included now) and battery but it is still pretty small and light.
No bells & whistles, just a “Basic Solution” to get things done.
shop.dronetag.cz
I am using the db120 because I could get it and it is very easy to use, if not a little clunky...