There are many factors that influence how much your rain garden will cost. The largest factor has to do with your ability to do the soil removal yourself. If you can dig your garden by hand, and your soil does not need any amendments, your only cost will be for plantings. If you need to hire a contractor to do the work for you, costs will be higher. If your soil is poor and amendments are needed, costs will be higher. Another factor will be the size of plants. Small plugs can be cheaper, however they will take longer to get established.