According to the Direct Marketing Association's "The DMA 2004 Response Rate Report," if you want responses then use the telephone.  But if you want to be cost effective and have a limited budget, mail is your clear choice.

The DMA study calculated the following as average response rates for each medium:

      Mail, including direct mail and dimensional mail:  3.2%

      Newspaper: 0.15%

      Catalogs: 2.23%

      Radio: 0.31%

      Magazine: 0.30%

      Direct Response Television: 0.14%

      Email: 2.31%

      Telephone, typically a follow-up to direct mail: 5.45%


The same study considered Revenue-per-contact and cost-per-contact from direct orders (medium/RPC/CPC):

      Mail, flat: $11.36 / $0.56

      Mail, dimensional: $14.16 / $0.91

      DRTV: $0.02 / $0.03

      Catalog: $1.48 / $0.69

      Radio: $0.08 / $0.07

      Magazine: $0.22 / $0.05

      Newspaper: $0.45 / $0.91

      Email: $1.60 / $0.10      

      Telephone: $45.37 / $2.50


Related sources: http://www.expoweb.com/Trends/may20054262005124902PM.htm

and http://www.usps.com/dminfo