The price for just the hardware (no licenses) starts at around 200€ (their just released CX7000), going all the way to several thousand € for their 32-core machines. Say for their midperformance CPUs (like the CX5140), it starts somewhere at around 1000€, all depending on the configuration. In software terms, what you pay for is then the runtime license. The higher the performance of the CPU, the higher is the license cost for the runtime and additional libraries. But to answer your question, yes, they have embedded PLCs in the 300-500USD range, but then you are on their lower end of performance (which is still not bad I would say, depending of course on your application requirements). Because they are PC-based controllers, you get very high performance even with the lower spec based ones.
For that price-range I would suggest you to start to look at their newer lines of PLCs:
CX7000:
https://www.beckhoff.com/english.asp?highlights/cx7000/default.htm?id=973374299344013
CX8100:
https://www.beckhoff.com/english.asp?highlights/cx8100/default.htm?id=97337477619629
Not sure the CX9020 would fit that bill:
https://www.beckhoff.com/english.asp?embedded_pc/embedded_pc_series_cx9020.htm?id=97337455631
I've personally recently bought the CX8190 to play around with, and it's quite a nice little PLC. Another good thing with the lower performance CPUs (from the price perspective) is that the licenses cost less than the more powerful siblings.
With the lower performance CPUs you normally don't get a full-blown Windows (like Win7 or Win10), but rather a slimmed-down Windows (like Windows CE). In the CX7000 line it's not even Windows but something Beckhoff calls TwinCAT-OS (which I've yet not figured out exactly what it is... RT-patched Linux derivate?).
However, if you just want to try it out, you don't have to spend a single cent as the runtime is free for seven days, and once the time has elapsed, you can renew it for another seven days like this for infinity. The development environment is also completely free.
