Before going for MEK1 or ME11, have a thin line difference between PBXX and PB00.
PBXX condition type is used for Manual Pricing in the purchase order where you enter price manually in the purchase order as PBXX condition type does not required any condition record(MEK1) or a inforecord (ME11).
PB00 is the condition type used for automatic pricing in the purchase order which is determined via access sequence which you can view in t.code:M/06 and have a inforecord (ME11) for material which will be entered in purchase order.
You can customize a new condition type( Copying PB00 condition type) and keep inforecord (ME11) or condition record(MEK1) for it!
For more refer a thread:
PBXX is manual condition type and it will not have access sequence assigned . So you will not be able to create condition record in MEK1.
PB00 will have accesss equence maintained , so you can maintain the condition record either in ME11 or in MEK1.
Check the condition types in config for both PB00 and PBXX under view changes which can be made chk for manual entries options you will have clear idea.