Those pesky ants! UGH! Well, here is the cheapest and most effective solution I have found:
In the microwave, melt a generous tablespoon full of honey or corn syrup. Stir in a teaspoon of Borax powder until it dissolves.
Using a yogurt lid (or something similar) place the mixture on it and sit it on your countertop or wherever you are encountering the critters. In a short time, you will have a circle of ants surrounding their 'banquet'. You may have to repeat the process a couple of times if it is a large colony of ants.
This works because they carry the mixture back to their nest to feed the larvae. It will kill both the ants and the larvae.
Borax is readily available (it's one of the ingredients in the Homemade Laundry Soap in an earlier post) and the cost is minimal for that and the bit of honey of corn syrup used.
If you don't already have Borax, you can buy it in the laundry detergent section at any grocery store. It makes a great laundry booster too, even if you don't use it to make your own detergent.
The total cost truly isn't more than a few cents and it really works.