The typical ratio for a one coat stucco mix is basically one part cement to 3 parts sand and about 3 gallons of water, however, there are many one coat stucco mixes out there today that have specific formulas.
There are kinds that only require you to add water to and some that require both water and sand. Typically the ones that already have the sand added to them are a lot more pricey (since you are paying for the convenience). Mixing the sand and water in yourself is the best way to do it, especially if you have a larger project to take on.
The Two Types Of Products Out There:
As I mentioned before, there are two types of one coat applications that you can use and deciding which one to go with is dependent on a couple of things.
Water Only Mixes:
These mixes only require water to be added to the dry contents. This method is more expensive and is roughly $10-$15 in cost per 80-90 pound bag.
The average square footage that these 80lb. bags cover are somewhere between 20-24 square feet at 3/8 of an inch in thickness.
So for smaller patches, this type of material would be ideal and a lot more convenient than renting a mixer and having to add your own sand into the mix as well.
Adding Both Sand And Water:
Just for the sake of argument, we will use the same 80 pound bag of stucco, from the same manufacturer in order to do a more accurate comparison. So, the same size bag that requires sand to be added yields about 73-80 square feet at 3/8 of an inch thick. The average cost for the bag of cement is somewhere between $8 and $10.
These are basically custom made mixes in which you add a certain amount of cement, acrylic and anything else you want to throw in the mix. Each manufacturer has their own recommendations on how to mix their particular one coat stucco, but all of their formulas are very close to one another and can be modified to suit your individual needs.