To begin to answer this question, you must first find out how much insulation you already have and then determine how much more would be cost-effective.
Many older homes have less insulation than homes built today.
A qualified home energy auditor will include an insulation check as a routine part of an energy audit. For information about home energy audits, call your local utility company. State energy offices are another valuable resource for information. An energy audit of your house will identify the amount of insulation you have and need, and will likely recommend other improvements as well. If you don't have someone else inspect your home, you'll need to look for insulation in several places.