As for the costs, those are normal in the publishing world. Many magazines have tried slashing the contribution per-page cost, and ended up with poor quality content as a result. We want the best contributors, and they're worth paying for.
100GBP per page may seem like a lot, but that's for 800 words of highly technical content, that has to be correct, well-written, informative and entertaining. A single page can take many hours of work when you're dealing with advanced subjects. You get what you pay for.