I was looking around at the time to make sure those were the best rates I could find. They were, but there were some cheapy hotels nearby at reasonable prices.
Try Premier Inn
and type "Birmingham NEC" in the hotel search (the urls don't retain search info so can't link directly, grr). They have, at time of posting this, a twin room for £180 for the three nights of the convention. No singles, by the looks of it, unfortunately. It's just over a mile away (see Google Maps
The Holiday Inn, Birmingham Airport
has (again, at time of posting this) a twin room for £60 a night over that period. It's about two miles away (Google Maps
The convention site mentions free shuttles from the airport to the hotel, I don't know if you would be able to take advantage of these maybe.
You could also try emailing the organizers (email addresses on the site, including one for the "Hotels team") and asking their advice.