Toilet tank dripping water condensation
A cheap solution is to increase airflow around the toilet. Place a small fan where it's directed toward the tank. By increasing the rate of evaporation less dew will form. 

There are insulated tanks available which have foam linings. You may be able to find one compatible with your stool. 

The converse of a tank insulated on the interior would be to wrap the tank with something that would insulate, such as a fabric. You could fit a towel or cloth around the tank and fasten it at the rear. 

Otherwise, if your plumbing is accessible, you could tee a hot water line into your cold water toilet supply and, by way of a valve, deliver a small amount of hot water to the toilet, raising the temperature. You'd just have to get it above the dew point, which is rarely over 75F.