As I said, it is not to do with weight but effective focal length. Longer lenses need better support.
http://www.reallyrightstuff.com/s.nl/it.I/id.15/.f
The BH-40 with LR plate comes in at around 220, the BH-30 at 170, so is the kirk BH-3.
The actual weight the head can support is one of the most meaningless variables. When you see numbers like 50kg that just means the minium weight that is guaranteed not to break the head, not the weight that is well supported. More important is the control, ergonomics, reliability. E.g., does the friction control knob operate smoothly and predictably, Can you rotate into a vertical orientation easily, how secure are the quick mount plates, is the head compatible with L-plates? Do they use arca-swiss compatible plates.? When you lock the head is there any movement at all or does it remain exactly as you tighten it (this rules out pretty much every head I have tried)
There are likely cheap heads that are sufficient, all depends how much compromise you want. I'm not familiar with some of the cheaper ones so someone else might give some suggestions.
I can say that my BH-55 is probably the most flawless piece of camera equipment I own. Every lens, camera, filter has some slight issues, but the BH-55 is just perfect, wonderfully engineered and I feel worth every dime. It is seriously expensive, but far cheaper than all the cameras and lenses I own and will outlast all of them by decades.
