Varies a lot by where you are - the southwest where stucco is common has much lower prices on the average. Typical cost range probably about $3-5/SF for a skim coat - an overlay which requires sandblasing to remove the existing paint or you are almost guaranteed to have bebonding and popout problems in a few years. Another $2/SF range for the painting.
One thing to bear in mind is truly "smooth" coat stucco does not have the life of sanded stucco, because the roughness provides a lot more surface area for wicking moisture to the surface and evaporating it. If desiring a truly "smooth" cost (rather than just "flat" sanded stucco), I would recommend considering one of the"synthetic stuccos" - which are actually an acrylic material, though you have to be sure to keep it well painted because it is sensitive to sunlight exposure.
Do NOT buy off on someone wanting to urethane coat or seal the surface - that will trap moisture in the stucco and cause progressive failure, as well as possible wall decay. Stucco needs to be able to breathe, and you should not have a water barrier on the outside of your house unless the house is designed for that through the entire wall thickness - and generally that is done only in extremely wet, rainforest and constantly foggy/rainy coastal areas.