If you keep reptiles, it's important to know the right temperatures and the amount of light required for the species. There's no shortage of specialty products available to heat reptile cages, but there are cheap alternatives that can work just as well. Don't compromise safety in your quest to find an inexpensive heating source. You'll need thermometers in or around the tank to monitor the temperature in the particular area.
The cheapest way to heat a reptile tank -- although you'll need additional heat and light sources -- is via natural sunlight. In nature, the intensity of sunlight and the time of year govern reptile activity levels and breeding behaviors. Give your reptile as much natural sunlight as possible, taking into consideration that direct sunlight can cause the tank to become too hot.
If you have a spare lamp around the house, you could put it to use heating your reptile tank during the day. Of course, that's dependent on the lamp's configuration, so that the bulb can heat the tank properly. Although incandescent bulbs are being phased out by manufacturers, they are available. The higher the wattage, the greater the heat factor. Take care to ensure that your lamp is rated for the bulb wattage you use. Adjust the light correctly, so that lower watt bulbs are closer to your pet, while high watt bulbs are somewhat farther away. Make sure your pet can't actually touch the light. If there's no screen on top of the cage, use a protective cover so your animal cannot touch the bulb.
You can purchase heating pads designed for reptiles, or you can use a less expensive human heating pad to warm the bottom of the tank. One advantage that human heating pads have over the reptile variety is that they are much easier to remove. They're much easier to clean and disinfect than reptile pads, and most have a choice of heat settings. Human heating pads don't suit all types of reptiles, but you can use them round-the-clock if keeping temperate snakes and at night for diurnal lizards.
Depending on the type of reptiles you're keeping, raising the temperature in the room can keep the cage sufficiently warm. While raising the temperature doesn't involve purchasing additional equipment, it's not necessarily cheap if the higher temperature significantly increases your heating bill. If you've got several reptile cages, it's probably a money saver compared to buying heating equipment for each one. An alternative is moving your reptile's cage into a main room of the house where the temperature is generally kept higher during cold weather. In the summer, you can use a small space heater to warm the area around the cage.