15 levels of the greater power were spent to reduce cost. 15 levels converts to three mastery points can reduce the cost to 0.
except the base cost was 6, so you reduced it to 3.
If I'm reading Greater Power right, and we haven't any houserules on the matter the cost is magnitude/2, or three to start with. Lesser powers are one per magnitude and personal powers are magnitude/2 again.
GTG now sorry about terse reply.
I'm used to lesser powers, so you are correct. Don't worry about the terseness.
I assume that Tayma is the girl in the delicate matter thread?
yes