seems like you'll need to get 2nd hand or 3rd party in order to get that price.
anyway, would you mind sharing what you need a external flashgun for?
From my experience, when covering events, the 400D's built-in flash will refuse to work after about 5 flashes about 1 sec apart. And it doesn't help the fact that I double tap my shots.
So an external flashgun helps as it can continously fire at least 5 shots in sucession, more than enough for me.
That means flash exposure compensation. Since most canon DSLR has E-TTL II, it will adjust the flash output to obtain a "reasonable" exposure. But sometimes, you wish to tweak the exposure setting, so that the flash output "underexposes" the picture taken.