One of my PCs wanted a tracking dog to help with tracking checks. I decided it would give him a +2 bonus on Perception checks to track. Since this is similar to thieve's tools (+2 bonus on certain Thievery checks), I ruled it would be the same price (20 gp). Since then I've decided that any piece of equipment that gives a +2 bonus to some subskill should cost 20 gp. (The climbing kit only costs 2gp but it is the exception that proves the rule.) So if someone wanted a healer's kit, or a disguise kit, or a survival kit, or whatever: 20 gp.
I gave the dog the stats of a wolf (level 2 skirmisher) and decided that commanding it was a minor action. But I told the player that if the dog took part in combat that the XP value of the encounter would be reduced by the XP value of the dog (125 XP), so the party did a good job of keeping the dog on the sidelines. I also told them that no, they couldn't kill their own dog for XP.
-- 77IM