For $515, you get all of the obvious, plus:
A 'real' enduro using enduro timing and cards, which is damn rare now (rather than a Pony Express).
A solid event, with decent length days.
A decent challenge (although A4DEs are nowhere near as difficult as they apparently used to be).
Secure Parc Ferme.
Compare yourself to the big boys.
Depth of competition (even if you're slow).
Lots of experience.
I think my 2012 A4DE cost me about $2500 all up - but that's for EVERYTHING, including entry, fuel to get there, extra jerry cans when they sprung a '10 litre or smaller' rule on us at Rider's Briefing, accomodation, extras for the bike, food (which we were going to eat anyhow), etc etc.
Finishing a Four Day was a big deal for me, and I figured I was only likely to get one shot at it, so I didn't take any chances - there were plenty of non-essentials that I happily paid for, just because they reduced the chance of a DNF. Turned out that I didn't need to use any of them... I reckon I could have done it for $1500 (remembering that it was an 8.5-hour drive from home for me).
I was keen enough to go over to WA, but it clashes with the 24 hour this year - and the relative simplicity of going to SA over WA means the A4DE lost out.