Yes mushrooms are expensive. You can add button mushrooms to any exotic mushroom dish. I am not recommending this, just telling the truth. Think of them as sponges. If you leave them in water, their flavor will be diluted.
Purists use just a brush.
You can hold them under tap water just before you cook them or dry them gently with a paper towel and slice them. The problem for storing fresh mushrooms is air circulation and drying out. Do not keep them in a plastic bag or other sealed container. Put them in an open container such as a strainer with a damp paper towel over them. A paper bag will also work.