I can back up that comment about the cost of the gear. I was told a figure of between £500-600 a while back. Seems a lot, seeing as the speed box seems to overheat perpetually.
The thing to be aware of is that they can tweak your settings to some degree when trouble-shooting, but only so far. In my case, they reduced the packet size and I suddenly stopped getting so many dropped packets and things became more stable. They may also try you on more than one aerial at the base station.
You may get 512k all the time, but then again at 13km range you may only get 256k (better than a kick in the teeth). You need to monitor things over a week or so to get a realistic sense of what's going on at certain times of the day or night. I certainly get degradation at peak usage times, but at others I peak up to the 450/500 range.
What I can recommend for checking your connection out is the following widget... <a href="http://www.webattack.com/get/netmedic.shtml" target="_blank">http://www.webattack.com/get/netmedic.shtml</a>
I've been using this little guy for years. Most of it's pretty useless, but the bandwidth throughput monitor is really useful for keeping an eye on your connection rate.