Realistic expectation of buying a used vehicle

I am about to start looking at buying a new vehicle. I am thinking of going used, just to get a few more features at a lower price. I am looking to spend in the area of 40k-50k. how much can I realistically expect to get off the asking price? I have seen people on the internet claiming they buy their used vehicles 20% lower than asking price, and that just seems crazy to me. I would love to get that much off if I can. If anyone has any suggestions, please share.

Thanks in advance.