Looked at Digikey to get them as I needed a few to solder on board and saw that those with less memory costs more than one with more memory. Why?
Supply and demand.
We are discussing what is essentially a "commodity". And one that is a little long in the tooth.
The non-P versions seem to all cost more money. Probably due to lower volume and/or Atmel/Microchip trying to nudge people towards the newer versions.
If you compare 168p vs 328p, the 328p is more expensive.
A manufacturer once reminded me that cost and price are not necessarily related.
Nor are value and price. Economics is known as the dismal science.