If you have a distribution of work where most is easy and cheap, some is moderate and moderate, and a little is difficult and expensive, and you take out all the cheap and easy work, the moderate and difficult work could drop in price but the average of the remaining work will still be higher than before.
e.g.
You have tasks advertised in the distribution $1, $1, $1, $1, $1, $1, $2, $2, $3, $3, $5, $5, $10. Median price is $2, and average is $2.76.
All the $1 and $2 tasks are replaced with AI. Old tasks get $1 cheaper each as there are more people that can do them. Now the distribution is $2, $2, $4, $4, $9. Median is $4, average is $4.2.
So you have made labour less valuable but the prices advertised go up because only the more expensive work now gets advertised.