Hi Norbert ,
we make an extension to manage in this way :
a) standard way (pricelist is mandatory ) 
b) price list is set to zero (on the fly )  
c) price entered in the PO is set as Price List if not found  (on the fly ) 
on the BP you have a flag is the Vendor follow these rules or must follow the standard
This is usefull on PO but it works also on SO.
Now we are extending the replenish report . We would like to  creating PO order also if the price is not set (now get an error ) . 
Many times you set the default  Vendor  but you don't have the price at that moment .  Now You could send the PO without price in draft as a "RFO"   and get the last price from the vendor .. you fill the Po and set the price list..etc
Marco