When a villager is converted to a zombie villager several times the prices also go down several times, however after trading with them a few times (sometimes it takes quite a lot of trades) the price will go up again temporarily, the price resets to the proper value at the start of the next workday. 

For example:

A librarian is converted 4 times and sells 1 paper for 1 emerald, after using this trade several times the trade will go back up to 18/19 paper for 1 emerald as if the villager was converted only once. At the start of the next villager work day the trade will go back down to 1 paper for 1 emerald

Not tested on 1.15 but it probably works the same way