We are working on several decorative plaster jobs right now and the materials range from $30.00 per gallon to $80.00 per gallon. You also have to account for the walls to be primed, sanded, areas around covered, the time to hand apply the plaster with a trowel, then dry, then polish to the desired sheen, then apply a protective finish.
The cheapest Venetian plaster is from the local home improvement stores. They are usually an acrylic based product, harder to apply, harder to polish.What you save money for materials you lose in time to do the work.
The expensive plasters are lime based. We have to use a special primer for them to make sure they bond to the surface and to make sure the lime will not eat away your walls. They apply to using warm butter and polish to a mirror-like finish with little effort. They are amazing products. The better products are from one of our favorite stores Chicago Institute of Fine finishes in Carol Stream. Aside from an amazing array of products to use the knowledge of product usage is superior to anyone else. They are there to help make your world beautiful.