Another of those things that's so natural to horse people we often forget to tell others.
The weight of a saddle is highly variable. English saddles weigh less than western saddles.
A typical English saddle weighs about ten pounds.
A typical western saddle weighs 30-40 pounds.
Roping saddles, which are designed to have extra structural integrity when the steer is pulling on the saddle average 50 pounds.
The heaviest modern saddles are western parade saddles, which are covered in silver and weigh 75 to 100 pounds. (The horses are conditioned to carry the extra weight and not asked to do it for long - but I don't envy the cowboys. I don't think I could get one of those on a horse!)
The lightest modern saddle is obviously a racing saddle. There are racing saddles on the market that weigh 10 ounces.
If you're looking at historical saddles, then I'm unable to find great information, but Medieval saddles were light, similar to English saddles, albeit in a quite different design. The McClellan saddle is known for being very light.