Last time I checked, you could find the HD-650 around $325. Check with HeadRoom and TTVJ. Both are authorized dealers and both contribute heavily to Head-Fi. They deserve your support and their prices are always closest to the lowest or worth it anyhow just for their excellent customer service. A couple others I've bought from are B&H and J&R. They don't sponsor Head-Fi, but they're excellent, too, and have great prices.
As for the headphones, I prefer the HD-650 by a wide margin. First, I think open headphones are superior, generally. The only closed headphones I think are good are the Sony R10 and the Beyerdynamic DT48. The R10 is probably a little more than you want to spend and I wouldn't recommend the DT48 to you - it's a niche headphone for hairshirt acoustic freaks. I like the Sony MDR-7509HD, too, but it's not in the same league as a HD-650.
I've tried Denons several times and I do not like their bass. There's too much and it's poorly controlled - modded or not. The bass detracts from the mids and highs, too. Some people like that sound, but the HD-650's presentation is much more balanced and precise. I'd rather be able to pick out individual bass notes and have them blend seamlessly with the rest of the music. Also, the HD-650 has a better soundstage and presentation than any closed headphone, save maybe the R10.