There are a number of NDI PTZ cameras on Amazon at the moment about 1/2 price and appear to be PTZOptics knock-offs. Whether those items are 'legal' or not, I'm not sure. Certainly appears unethical copying ... but maybe not. However, the primary one I looked at has a gmail support address. These are fly-by-night operations, with (very) poor English translations, and bugs, and not certainty of getting future firmware updates (ex updated NDI versions).... so, as much as I'm sorely tempted for a 2nd NDI PTZ at our non-profit, I'm not sure 1/2 now represents good value.... As for finding used, last year this time there were wait times for the cameras. So, I wouldn't expect much savings on used (but I haven't researched that deeply). Then again, a serial PTZ might well be available for lower price, but then you have to consider the overall price (as you then will need both the serial cable, as well power, and SDI, HDMI, or ?? cabling, and a capture card, etc... why we went NDI).
If buying used, my approach would be to buy form a Tier 1 vendor, where you can get support (and possibly repairs) if need be
If the streaming PC and a single camera are close to each other and no plan to expend to multi-camera, I'd be inclined to a USB PTZ camera for simplicity