This is a very simplified example but hopefully it will help:
Assume that there are 10 nobles and 100 workers. Each noble has 10 fields and uses at least 1 worker per field.
Nobles have financial commitments and a standard of living which they wish to maintain. They pay the workers 1 pound a week to work on the fields and each field is equally profitable.
Workers cannot easily move around as nobles elsewhere are paying roughly the same and already have manpower so the wage is fairly static. The good and sundries that workers can by is within their price range as there is no point charging more as they will not be able to buy it.
Lets assume that your calculation about the ratio of deaths is equal and each class loses 50% of its members.
There are now 5 nobles, 50 workers (25 of which have no nobleman to work for)- each noble still has 10 fields which requires at least 1 worker per field.
Now each noble now only has 5 workers and therefore has 5 empty fields and will therefore make less money but they still have their commitments and standard of living which they wish to maintain.
There are various workers which no longer have anywhere to work but still need to feed
Nobleman #1 is called Alfred has the idea that he can pay these people to come and work for him. Nobleman #2 is called Bill and has the same idea.
Alfred offers them 1 pound to work on his field, Bill offers them 2 pounds. The rest of Alfred's and Bill's workers realise that they have power - they are a scarce resource so they contact noblemen 3,4,5 (Charles, Dave and Edward) and ask how much they would offer. Edward realises the value of having more than 1 worker per field now that they can buy them on the free market and begin buying up the finite labour source as an investment as he had the backing capital to do it, unlike poor Alfred.
Eventually Alfred manages to persuade 6 workers to work for him at the extortionate price of 6 pounds per week. Hence he was paying 10 pounds a week for 10 fields worth, he is now paying 36 pounds a week for 6 fields worth. This is awful and he cannot afford to buy that new set of armour. He tells everyone who will listen how greedy and lazy the workers are.
The workers who work for Alfred now have 6 times the buying power they had before so they go to the market/tavern/whatever and buy everything in site. The next week the owners of the market have felt the effect of the increase in cost of workers and have to charge more for the goods to make a profit hence everything becomes more expensive. If the market owners think they can get away with it they also tag some extra on the increase to make some extra money.
Suddenly Alfred's workers are not that rich in real terms and demand a pay rise or they will leave and work for Edward who is desperately offering massive amounts of money to anyone who will work for him.
However, everytime they get a pay rise to buy more things the cost of making things goes up (as their wages are part of the cost) and an inflation spiral begins.
Due to the massive increase in prices at the market and the cost of labour the noblemen find that they are not able to make as much profit as before and struggle to afford to pay the worker enough to keep them let alone maintain their standard of living. Hence they are far worse off.
Workers are better off but the inflated costs of good is crippling some people who cannot get the high wages for whatever reason.