Diatomaceous Earth (DE). While researching for ant, and termite remedies, I came across this solution. Generally used for swimming pool filtration.
We don't have red fire ants where we live, and have not tried it. Terro liquid bait mentioned above for small black ants is like "ant crack". A little drop on the beginnings of an ant trail and bam! No more black ants!
If anyone tries this cost effective solution, I would be interested to know if it works on red ants.
Do a Google search for various info using diatomaceous earth.
http://www.ehow.com/facts_7169952_diatomaceous-earth-kill-fire-ants.html