Espresso is the best for preventing withdrawal. You can buy espresso cans like these. They end up costing around $1-2 per drink.

You can also consider buying a moka pot. Though, moka pot coffee isn't really espresso as it is extracted under much less pressure. But you can make your own and so save money. It tastes quite good.

A single shot of espresso from Starbucks, costs around $1.50. If you want something strong, I recommend a red-eye: a cup of drip coffee with a shot of espresso in it. This usually will keep you caffeinated for a few hours at least.