By installing a programmable thermostat you are allowing yourself the capability to lower the temperature while you are away. Being able to lower (or raise) your thermostat while you are away so you are not paying to heat or cool can save you a considerable amount of money. You can get a programmable thermostat for as low as $30, although some systems may require a specific thermostat that costs more money. Regardless, you will quickly recover your costs by being able to program your thermostat. Some models will even allow you to program it so that your heat or central air come on within a specific range rather than kicking on every it drops or raises a degree.
Also, check with your local utility company. Some offer a rebate incentive to cover a portion of the cost of your thermostat. Our electric company will give us a rebate of up to $20 and I have seem them offer this rebate the last 3 years, including this year.