I was a tent camper for many years myself and can appreciate what you are investigating. Lowering the temperature inside a tent with electricity is not an easy task. However, here are a few pointers for actual tent setup that might help.
1. If there is a tree nearby, try to setup under the shade. It is much easier to maintain lower temperature from the beginning as opposed to trying to lower them when it is already hot.
2. In dire straights, I have even setup an easy-up over my tent.
3. If you are not worried about privacy, remove the rain fly. This helps the hot air escape. Rain flies are easy to take on and off, especially if you leave half of it still attached to the tent.
4. Tents are waterproof, both inside and out. Avoid anything that creates condensation. A battery operated handheld fan hanging from the top of the tent will help sometimes too. Don't worry so much about blowing cold air in, but try to exhaust hot air out (either through the top of the tent when the fly is off, or through the door.
Ok, for your questions:
-I desert camp in Anza-Borrego in the fall and winter, so it ranges from 80 degrees in the day to freezing temperatures at night. Usually for three days.
-I would spend up to $50.