$2.20 on some websites and no shipping cost
36 cards would cost £1
The Auzentech X-Fi Prelude will be better in games than the all ASUS cards, but it will also cost nearly twice as much.
Yes -- sound cards are available for a wide range of prices from about 10$ to over 1000$
Yes -- most modern sound cards consume much less power than before.
It depends on what kind of cards they are.
The cheapest sound cards start around 30-40 USD. Of course there are really pricey cards aswell, for example the RME HDSP 9632 Hammerfall. In general the a sound card cost between 30-450 USD
You can buy Firewire sound cards, but they do cost a bit more these days.
It depends where you buy it from.
£1.00
$10
It is around £20 ($30)
$1