The cost is about $6-$8 square foot. That means measuring the length by height of all walls. This will give you a fairly accurate answer. The guys who advertise through mailers or leaflets are a lot higher. Get a company that specializes in stucco and plaster only, not a general contractor so to avoid a 25% markup. Keep in mind if you have curved, odd shaped walls or add "foam shapes" this increases the costs.
Costs can vary depending on the geographical area you live in.