Skeet and affordable really don't belong in the same sentence.

It really isn't a "skeet" specific gun but the Beretta 3901 statesman with a 28" barrelwill get you going with out breaking the bank too much. They go for about $700. It is a good solid semi auto with a very simple and very reliable gas system that uses no o-rings to seal off gas. It also makes a great dove gun.

http://berettausa.com/product/product_field_guns_main.htm