Hi,
At present your material have stock and why you want to have different price for same material?
Better active split valuation for your plant & create a new material for plant by entering valuation category and have your valuation types to have different price's for same (new)material
Now do material to material transfer form old material number to new material number with t.code:MB1B with 309 movement type. Finally set deletion flag to old material number.
Regards,
Biju K