Finishes that are done by the concrete finisher, or maybe one or two extra workers on the crew, are going to be the least expensive of the decorative concrete ideas listed here. The reason is because they require few added materials to produce (except for maybe several bags of salt or a broom). When special tools are required, like stamps or stencils, the price of your concrete project will rise.
Many people believe that stamping, exposed aggregate and even broom finishing require fresh concrete. But with concrete resurfacing you can often get the same finishes even on existing surfaces.