As far as I know, there aren't any rules for including feat effects in magic items, although there are several magic items that allow the user to act as though they had a particular feat. Usually, said feat is a prerequisite for crafting the item.
Off the top of my head, I'd say a single cost modifier to the item would be acceptable. Maybe around 5,000 gp to start. Perhaps more or less depending on the feat.
Really what you need to consider is how it'll affect the balance of your game. Is Point Blank Shot on a bow worth a +1 enhancement? Maybe. Are PBS and Precise Shot together worth +3? Probably not. Are they worth 10,000 gp? Not at low levels, definitely at high levels.
That's the main problem: adding feats to items makes the magic item pricing rules even more fickle than they already are. Not only that, but you'll have to decide whether having a feat item qualifies as a prerequisite for your characters to take other feats (a subject frequently and vehemently debated on these here forums).