Depending on the case that you are using, there are several quality alternatives. If your case is plenty wide, the best choices would most probably be a Scythe Ninja with a quiet 120mm fan or the Big Typhoon with a Panaflo M1BX or M1A, but both are tall as hell and have trouble fitting into some cases. The Zalman 9500 is also a good choice and a little shorter than the first 2, but my experience with it shows it's not quite as good as the BT and Ninja. The XP90C with a Panaflo M1BX is at least as good as the 9500 and I'll be trying it out with a Panaflo H1BX, Delta EFB 92 X 38mm 3 blade and a 92mm Vantec Tornado next week when I get in from work. I'll be running the Delta and Vantec on a fan controller so I can vary the speed and noise to see where they hit a happy medium between noise and airflow. Aslo, the stock 4 pipe retail hsf that comes with the Opteron DC procs and X2 4400 and above procs is real good with an upgraded fan on them. I have tried a Delta FFB0812HHE on one of my retails and it worked as well or maybe a little better than an XP90(not the copper XP90C) with a M1BX. I think that the 80 X 38mm Delta EFB 3 blade fan that sidewinder sells would also be a good choice for the retail hsf too.
As far as prices go, I got my Xp90C off fleabay for $35 shipped and I bought my Ninja from the Newegg refurbs for around $30 and got my 9500 off a guy at the Short Media forums for $45 shipped. The only one I paid retail for was the BT from Newegg. It was around $50.