Sorry, you're right. I did a mental currency conversion from $1200 aussie dollars, but the pound seems weaker these days. sorry! I changed that the original post.
You don't need the external antenna. In fact, the best setup you can possibly have is to set up the SBS-1 in a box on your roof, use the supplied magmount antenna (mount it to your tv antenna for instance, vertically of course) and run a 12m USB cable to the box from your computer. I don't know how long the USB cable can get, but there are extenders that run USB over ethernet cable, and I've heard they can be very long. I'm running 12m without any problems (and without additional power supply, just USB).
I'm getting excellent range with that setup. You can also add a £75 preamp to make things even better.
Cheers