The latest announcement doesn't list prices, but says it'll be similar to the Paris conference this year - which was a bit less than northwing says. Paris was Eu300 for non-members, Eu275 for members, and a bit more for the social events, in total you could have just about spend Eu500 (£350ish) for everything except accomodation.
The RAeS is a great organisation, but it does treat it's conferences (and even more so those by other organisations using RAeS facilities) as fundraising ventures far more than many of us think is healthy. Much of that money is almost certainly going towards the stiff running costs of 4 Hamilton Place. Is this position defensible?, many think not, but unfortunately the many aren't running 4HP. RAeS own committees have to fight tooth-and-nail to run a conference as cheap as £80 /day (and those will be what 4HP calls "class C" with very little society support or publicity to it.
G