I'm looking for an inexpensive beam mount rack for light commuting use. Something made of aluminum, or an alu/plastic combo. Around the $20 to $25 price range, a load capacity of 9 kilo/20 lbs. minimum and, preferably, round edges so I can mount a light on it.

I checked out the one from Zefal which is thermoplastic with aluminum support tubes, it is quite nice and feels sturdy but is too short. I also checked out Topeak but they are in a different (higher) quality and price range, which is unnecessary for my use (they do look very nice however ). Also tried BBB's Postrack, reasonable quality but didn't feel as sturdy as the Zefal. Plus it's practically impossible to mount a light on it.

Anyone have any suggestions? Thanks in advance!

Sure,
buy a real rack. Planet Bike makes a good one for $25. It's light, strong, and cheap.

I thought about that. It's just that my bike is a MTB and I would like to maintain that typical MTB look, if you know what I mean.

Check out the Performance, Nashbar, and Supergo sites. They all have quick release as well as "take off the bolt" beam racks.
When I saw the title of this thread I thought you'd found some kind of beam that would clear the lane in front of us for cycling. Hit the switch and vaporize everything.

I know you mentioned that the Topeak was out of your price range but IMHO, that's really the upper-end of beam rack quality. I have one and it works great for around 15 lbs. of cargo but I would definately not go much above that and I would definately not use it for anything other than picnic runs on smooth roads or bike paths. Also, you'll want to keep your bulk down and panniers are definately out. Do you have a full-suspension or a hardtail? If it's a hardtail then I'd recommend a full rack.

It's a hardtail. The reason I prefer a seatpost mount rack is because IMO it looks better then a full rack as it doesn't alter the typical MTB look too much. I saw on the Topeak website that they now have an aluminum/composite version of the QR Beam. The main tube is still alu and the plate is composite for lighter weight. It's rated for 15 lbs max. It's slightly above my price range ($29.95) but still cheaper then the $49.95 the all-aluminum version costs. Seems good, but gotta check it out in the shop first. Hmmmmm.

Btw khuon, some pretty bikes you got.