Thanks for the response! While a pro model with 3 or more 0s is off the table, I might just break down and get an Arinst, but I'll try the HackRF first.
A quick question. As far as quickly detecting Sub-GHz transmitters, would you recommend a TinySA? Would a RF Explorer be better?
For quickly detecting sub-Ghz transmitters you can use an RTL-SDR. They are dirt cheap. But you have to handle the software.
OP said he has a "Portapack", which is actually a add-on board for the HackRF One, that allows to use it independantly from a computer.
All OP has to do is to put the device in "HackRF One" mode and connect it to a PC running Windows. Then he just has to run the software I linked in my original reply (alternatively he could use Satsagen). This implements a pretty nice real-time spectrum analyzer (with some obvious limitations, though). This is the best he will ever get for his budget and in reality he does not have to spend any more money, as he ALREADY OWNS the required device!
Recommening an RTL-SDR does not make much sense, as it is worse than a HackRF One (2MHz RT span vs 10MHz RT span).
Also, why recommend an ADALM Pluto, if it is more or less similar to the HackRF in this application (I know it can simultaneously do RX/TX, while the HackRF is assynchronous RX/TX).
There are, IMHO, only two options:
1) Stick with what is already available
2) Increase the budget by around 20 times, to get something truly better