Soil amendments can be a substantial portion of a rain garden budget. Depending on the mix used, the garden will be either extremely well drained/dry with periods of flooding or generally damp with periods of flooding. 

This mixture will create a very well drained, fairly low nutrient mix for a rain garden: 

15% BY VOLUME DOUBLE SHREDDED HARDWOOD MULCH
50% BY VOLUME WASHED SHARP SAND – ASTMC-33
30% BY VOLUME TOPSOIL
5% BY VOLUME PEATMOSS

The Virginia Department of Forestry recommends homeowners use: 

50% sand, 25% topsoil and 25% compost or leaf litter (www.dof.virginia.gov).

It is important to note that the sand used in a rain garden should ALWAYS be a coarse, sharp sand. It should NEVER be a contractor building sand.   In areas of high summer soil heat, the compost or peat moss components will decompose rapidly.   The double shredded harwood mulch will decompose slower and will maintain the desired coarse and well drained structure longer than some other amendments.  In all cases, the quality of materials matters; topsoil should be weedfree and high in organic content and less than 10% clay.

Links:

http://www.lid-stormwater.net/soilamend/soilamend_home.htm

http://www.lid-stormwater.net/soilamend/soilamend_maintain.htm