The 25quid (15 at Wakefield) -includes- the setup of the forwarding account, so its not an extra fiver on top.
People with existing AAUG.net accounts can upgrade but it'll still cost 25UKP (as they only paid 1 or 2UKP in the first place!)
People who subscribe to the unfiltered package for 5UKP will be able to upgrade for 20UKP at a later date.
We arent using SpamAssassin, but we're not using something we've written outselves, its a fully supported, commerical package.
If important email does get flagged as spam (and I've been using it for a while, and its amazingly accurate), you can whitelist it using the web interface so that in future, it'll pass through unfiltered.
As jonix stated, this product isnt aimed at the sysadmin who knows their way round sendmail and spamassasin.
Most people just want email without the hassle - and this is a step in that direction.