a very cheap solution is to get one of those small high-rad tubes on ebay from the Ukraine for about 6$. I managed to get an output out of it with a small gamma source and a old HV-linear lab power supply
If you are actually doing experiments it might make more sense to get yourself a high voltage variable power supply and a bare tube.
I even managed to over-voltage my tube during experiments to get visible arc discharge from radiation (though this likely damaged it).
If you just want to learn about the interface electronics I would suggest you go for a HVPSU and a bare tube and build it yourself so you can get an idea of what changing bias does etc. I like the old keithley vacuum tube supplies.
Keep in mind though, they are earthground referenced by design. I guess they are scared to float it.
If you manage to find one for <100$ on ebay that is. I don't see any right now. I think I got a broken 1200V keithley reference supply for like 50$ and fixed it up with 12$ of tube replacements. Adding in a 7$ tube it came out less then the commercial solution, so long you don't mind using a scope.
Then in the future you have the supply and can use photomultipliers and more advanced tubes.
Keep in mind you want money for
1) radiations source of different type
2) fixtures (even wooden stuff designed to hold attenautors, detectors and samples and move them around on screw threads or such for precise distance control is fairly time consuming, interesting and professional.
3) shielding (i.e. a project could be to make standard size lead, wood, steel, etc shields of various thicknesses and dimensions to interface with your tube). You can actually buy old 'radiation attenuators' which are basically measured disks made of various materials used to calibrate equipment, i.e. if your source is too strong and full-scales the range, you put a radiation shield between the source and the detector to attenuate it by a known amount.
4) nice storage boxes that are safe to keep (custom casting a lead shield that tightly holds your sample in a nice wooden box comes out a bit pricey if you factor in things like hinges, good quality wood, wood/lead working tools).
Without these things I don't think you will be doing serious radiation experiments.
Having a high voltage adjustable power supply not only opens the door to various tubes, photomultipliers (with crystal detectors) but also alpha particle detectors, which operate in free air and are shop buildable :
https://carlwillis.wordpress.com/2011/09/17/a-simple-spark-detector-for-alpha-particles/ by fixture I mean so you avoid what that guy is doing, holding a radioactive source in his hands over a 8000V energized conductor. A nice fixture would slowly bring the sample towards the detector on a screw. And you would wanna put a ruler on it too.
And for serious testing, like determining if you have radioactive metal for your own safety, if you are scrapping for instance, I would not recommend using low cost chinese electronics.
As a side note, my 300$ Belgian made radon detector broke after about a year with firmware curruption errors. I don't trust these little 'gadgets' very much. Get something with a metal box for the electronics. I did however loan it to someone for a long period of time (something like 1 year measurements), they possibly mistreated it some how. Cannot verify but I would not spend much on portable stuff unless its really old/military spec.