Actually there are two mutes you can make yourself, that cost little or nothing, and do a good job.
You know those flexible-magnet refrigerator-door stickies? Black on one side and often an advertisement printed on the other? Get some of that stuff--with or without advertisement. Cut two strips, about 3/4 inch by a little (say a quarter inch) longer than the width of the strings just tailpiece-ward of the bridge. Put one strip under the strings in that location, black side up, and one on top, black side down. They hold each other in place and mute the banjo admirably.
Alternative approach: Cut similar-sized strips of Velcro, one from the pile and one from the hooks. Put one under the strings, the other to meet it on top of the strings. This also does the job.
Price? Cheap to free, depending on your source of material.
Efficacy? Pretty good.