"This is not centralised, there's just one single central entity that negotiates"

that's not really a contradiction to what I wrote above. The point of having one central negotiator is that this negotiator has a lot more leverage, because they control the demand of an entire market. If I tell a drug company, that they can sell me drug A for $X per dose or not sell any of drug A at all, you can except a lot more pressure on prices compared to negotiating only for one insurance plan among dozens.

Even the USSR had prices, and used them in it's planning.