I am looking for the least expensive source available via website for
2? x 3? full color signs on foam core. Each sign would be different
and we would be ordering 10-20 and having them shipped to Atlanta. I
have a local Atlanta quote at approximately $60 each, but believe I
must be able to do considerably better.

There is an alternative to foam core depending on the end-use of the
sign.  The security industry in Australia commonly use Corex
Fluteboard for exterior (gates, fences) signage.  This is a durable
plastic material that is very cost effective and give a good reaction
to printing.

Their website is: http://www.corex.net.au/

As you can see from the website, the material is also used for
packaging, storage, transportation, point of sale and a whole heap of
other things.

A price I got a couple of years ago for signage about the size you are
looking at was about $AUD30 per sign.

Cheers!