To change a stereo power supply from 115 volts to 240 volts iff poss approx how much onkyo?

As far as I can understand from your question, you want to know the approximate price for changing the power supply in some onkyo component.

It's not an expensive thing to do (in fact free in most cases). Just look at the back portion of your model and there should be a knob (usually for a screw-driver access) to switch this value. Some older models had this built inside and these models had to be opened and the engineer would put the jack for the power supply on the right plug (on the circuit board).