there is on hard and fast rule for putting feats into magic items because there is no hard and fast rule for determining what a feat does.
that said, there is an ad-hoc rule that has been mentioned, in passing, in several offical D&D rulebooks, including Arms and Equipment Guide and Magic of Faerun.
a feat that has an effect similar to an existing magic item has a price similar to an existing magic item. for example, Great Fortitude grants a +2 untyped bonus to fort saves. the cost of an individual save bonus is between 350 and 500 gold (sources vary). a +2 bonus would cost 4*400 (or whatever), *2 for an untyped bonus, so about 3,200 gold.
a feat that does not have an effect similar to an existing magical item is worth between 5,000 and 20,000 gold, depending on just how strong the feat itself is. an additional 10,000 gold should be added for each prerequisite feat (this does not grant the prereq and should, in my opinion, be counted even if the item also grants the prereq).
note that the above is an unoffical rule. feats are powerful, and part of thier value comes from the fact that they are so rare.
finally, be very careful about using 'item feats' to meet the prerequisites for other feats and/or prestige classes. it's perfectly legal to use any item property (including granted feats) to meet the prereqs of said things, but if the item is lost, destroyed, dispelled, etc., then ALL abilities dependant on said effects stop working. if you lose a requirement for a feat, then you cannot use the feat until the requirement is regained. if you lose a requirement for a prestige class, then you lose ALL FEATURES of said class until the requirement is regained. a feature, btw, is anything granted by a class other than BAB, save bonuses, Hit Dice, and Skill Points. also, you do not lose proficiencies, even though they are technically features.