1. Purchasing a furnace or air conditioning unit that is bigger than needed.
2. Not checking for leakage when installing a new heating and cooling system.
3. Only insulating the attic because heat rises. Walls, windows and floors over a basement or crawl-space are just as important.
4. Using only air conditioning to cool a house. Fans are more efficient, and use in conjunction with air conditioning on a higher setting can result in the same comfort level.

In a house with central air conditioning, the unit is probably the biggest by far. Although used only a few months, the annual cost is much greater than the annual cost of a refrigerator, which is typically the next largest contributor. In hot climates, the annual air conditioner cost can exceed a thousand dollars. To get a rough idea of what your air conditioner is costing you, subtract a spring electric bill from a summer electric bill. This gives you an approximate monthly cost. Multiply this by the number of months you use your air conditioner to get an approximate annual cost.
Refrigerators are typically the largest electric bill contributor in houses without air conditioning or where air conditioners are only used a few days of the year. If your refrigerator is more than ten years old, you should consider replacing it. New efficiency standards have been in effect since 1992, and older models are typically two to three times more expensive to run. Contact the American Council for an Energy-Efficient Economy list of refrigerators.

Moving air makes you feel cooler, therefore you can turn up the air conditioner thermostat or turn it off altogether when using a fan. Whole-house fans are a great substitute for air conditioning, since they move large amounts of air through the house. Independently, a whole-house fan provides good comfort levels when it's not too humid or hot outside. Studies by the Florida Solar Energy Center (FSEC) show that ceiling fans can save energy if the occupants turn up the A/C thermostat. Unfortunately, most people don't adjust the thermostat and leave them on even when no one is home, which can result in negative savings.
Research from the FSEC indicates that whole-house fan savings are quite variable, ranging from about 10% to 65%. This range is due to the effect of climate; a milder climate will see savings toward the upper end of that range. The FSEC also found that if the air conditioning thermostat is set 2 degrees Fahrenheit higher when using ceiling fans, the savings will be 14%. A higher thermostat setting results in higher savings. If the thermostat setting is not changed, electricity consumption can actually increase by 15%.

That used to be the case with fluorescent lights, but advances in technology, especially in ballasts, have resulted in lights that do not use appreciably more energy to start up. Turning fluorescent lights on and off does slightly shorten the lifetime, in hours, of the bulbs, but you will have to replace the bulbs less frequently if they are not running all day long. Incandescent lights do not require additional energy to start, in any event, so if you are leaving the room for more than a couple of seconds, you will save by turning the lights off, for both fluorescent and incandescent bulbs.

With rising energy prices, it's tempting to think about using portable room heaters. It's also worth considering a programmable thermostat. If your central heat is electric, you'll almost certainly save money by using portable electric heaters. In fact, part of the savings come because valuable heat isn't being lost in the ducts before it reaches your living area. If your central heat is gas, you might be able to save money by using portable electric heaters, but it's not something to take for granted.
Your local energy prices and desired comfort level will determine the answer. Also, a typical central furnace will provide about 20 times as much heat as a portable, depending on its efficiency. Keep in mind that even if the hourly cost of using portable electric heaters is lower than for your furnace, you will be dramatically increasing your home's overall power demand, which contributes to regional power shortages that ultimately can trigger blackouts and price increases.

What you're experiencing is a major problem with large appliances that require contractors for installation. There is no price list available in this type of situation as prices can vary widely by contractor or area. It is even difficult to get wholesale listings from manufacturers, since they may charge different amounts to different contractors. If a local home repair store carries the brand of furnace you are interested in, you can get a general idea of the unit cost. Unfortunately this does not cover labor and installation costs.
The best way to figure out if a contractor presents a reasonable bid is to shop around. Before you sign a contract, get several bids from different contractors. For each bid, get all of the details in writing, and make sure the bids are comparable. Be careful if one of the bids is dramatically lower than all of the others. Make sure that the same work is actually being bid on. In addition, check with the local Better Business Bureau and find out if any of the contractors have complaints against them, and get details about complaints if you can.