Jlane brings up a very good point. Most cow hunts are advertised as 3-5 day hunts but in reality end up being a 1 or 2 day hunt because they are held on private ranches with excellent elk populations and getting a cow is almost assured. An outfitter can do a hunt like this for $1500 or less while he couldn't do a 5 or 6 day pack in hunt for that kind of money. Plus the odds of getting a cow wouldn't be as good on a hunt like that.
There are also some outfitters that allow you to take a cow, in addition to a bull, for no extra charge.