Hi,
You may check the following options:
1.Even if you maintain the item as free item, the header level condition tab will be available. You can maintain a specific header level statistical condition in the pricing procedure and maintain the price in PO - header level - conditions tab. However, the net price will be zero.
2. Maintain the price in some free text fields in PO header / item. You can find few free fields like "your reference" in header level - communications tab. You may make use of these fields.
3. You may use the header text also as already suggested by Akash.
Option 2 will allow you to pull the same in standard reports like ME2N (with dynamic selection.
Regards,
AKPT