How much additional is the power supply?

Actually you can pick up a couple of good power amps on ebay for maybe 60 bucks USD.

But actually there are lots more expenses, as you know if you have done it. You need a preamp if you are currently using a receiver. You need to buy new speakers if you don't know what the frequency response curve of the ones you have are. You need to purchase an appropriate 24 db slope linkwitz Riley crossover device. You need a separate crossover for the sub if you have one. Finally you need a condenser mic and preamp and RTA program for your laptop, or at least a radio shack sound level meter to balance the different speakers and amps.

its a lot of fun but you have to gear up for it.

Eric J.