Tricky question
....
1x10% = 0.1 Photon Drop
Then considering people buy 5 Power Materia for a Photon Drop, then we can consider that 1 Power Materia = 1/5 =0.2 = 20% of a photon drop, so 20%/2 = 10%, so 1/2 = 0.5, which means a half power materia (that Is 0.5 of a power materia) Is worth 10% of a photon drop