I'd say it's too cheap at 30,000 gp, but that's a sort-of gut reaction; haven't had the opportunity to compare prices with the existing items.
The vest would allow you, as Lord Pendragon mentioned, a +6 natural armour bonus. If one should craft an item that gave just a +6 natural armour, it'd cost 36,000 gp and occupy an amulet slot. But, let's say, because of other limitations of alter self (for instance, it gives the character only extraordinary special attacks, and no special qualities), the ability is worth only half that price, so that's 18,000 gp.
The item also gives you a +10 unnamed bonus to Disguise checks. If it were a competence bonus (magic items usually grant competence bonuses to skill checks), the price would be 10,000 gp. Unnamed bonus would warrant a higher price. However, for simplicity's sake, considering the drawbacks of alter self, let's leave it at 10,000. Multiplied for secondary item ability, that's 15,000 gp. You also get the racial bonus feats and racial skill bonuses (if any).
So, we're already at 33,000 gp. Throw in the possibility to gain different types of movement (burrowing, swim, climb, etc.), and the possible special attack modes (troglodyte's stench, for example), and the price easily jumps to 40,000 gp.
Note that, according to the rules, you can't get any water breathing abilities (as that requires humanoid with the aquatic subtype, and your type and subtype remain unchanged).