W.S. PTA approved is the one which has been imported through legal channel and all the formalities have been completed. So definitely, the price is higher.
On the other hand, NON-PTA approved may be the one which has come here through Grey Import and all the legalities / formalities have not been completed. So, the price is lower.
Secondly, NON-PTA would most probably be, without any warranty. PTA approved set will definitely be with at least one year warranty.
These may be the differences in my opinion.
Now, Buying and using Non-PTA approved mobile phones might not be legal or Safe. For further details, one may have a look at main PTA website, or at least, following link:
https://dirbs.pta.gov.pk/about-dirbs
I hope, it all helps. Thank you.