You don't mention what guns you are interesting in quieting down. With rifles and handloading, it is easy to make them very quiet. With handguns, about the only options have already been mentioned: .22s with CP caps or something like that, and wax or rubber bullets powered only by a primer.
If you can legally own a suppressor, you might be surprised at what they cost. They are not nearly as expensive as many people would lead you to believe. The "hoops" you have to jump through are not extensive. The only real big stumbling block is getting the CLEO signature. Here where I live, it is no big deal at all. The hardest part is catching him in the office for his signature: there is no hassle from him at all. The rest of the proceedure might take an hour counting driving time.
However, I realize it is not this easy for everyone and totally out of the quesiton for others.