I have not noticed any real change in "prices" other than when I defeat raiders going after the village. That got me a discount. Otherwise I bring multiple villagers to maxed out trading levels and the "prices" for trading remain the same. I also found that the traveling traders really don't have anything worthwhile to trade. I fail to see exactly what their benefit is.
Are you playing the Java edition? If so, this is very odd. My villagers lower their prices all the time when I trade with them, however I never bothered to pay attention to how many trades I have to make or whether I have to trade with multiple villagers to make this happen. Not paying attention to what the trades are can be costly, however. Once I was merrily trading with a lot of villagers who had all lowered their prices until I noticed that one villager was charging me much more for a particular trade. Apparently the law of supply and demand also plays a role in pricing, so the price can go up for an article you trade too much even if you are popular in the village.