Dear Carrie: I have over $600,000 in CDs at 3 percent interest. I have a mortgage balance of $100,000 at 5.125 percent. Would you advise that I go ahead and pay off my mortgage or leave the money in CDs? -- A Reader

Dear Reader: On the surface, the answer to your question might appear to be a simple calculation. But in reality, the decision to pay off a mortgage can be more complex. So I'm going to start by posing a few more questions that you may want to factor into your decision.

For instance, how many years are left on your mortgage? How close are you to retirement? Does the $600,000 in CDs represent your complete nest egg? Since I don't know your specific answers, I can only give you some broad guidelines to take into consideration.

Consider the real cost of your mortgage.

You say your current mortgage is at 5.125 percent, but have you factored in the tax deductibility? Let's assume you're in the 35 percent tax bracket and your mortgage interest is fully deductible. In this instance, a 5.1 percent mortgage would actually cost around 3.3 percent. Almost a wash with the 3 percent you're making on your CDs.

Factor in future investing opportunity vs. risk.

As I'm sure you know, investments that carry the most potential for reward generally also have the highest risk. A CD is at the very low end of the risk/reward spectrum. So think about your comfort level. Would you prefer to invest your money in potentially higher-yielding investments? If you think you can do better than 3 percent and are willing to take the risk, perhaps paying off your mortgage isn't the right decision.

On the other hand, current interest rates on CDs are very low right now. If you don't want to increase your risk level and can't match the 3 percent you're making now as your CDs come due, taking the money and paying off your mortgage might make the most sense.

Determine your cash needs.

It appears you're in a very strong cash position, so liquidity may not be as much of a concern for you as it might be for others. A preference for liquidity might keep you from paying off a low-rate mortgage prematurely even if you can't do as well or better with an alternative use of the money. Diversification could play a role here, too, as you look at your mortgage in light of your overall financial plan. Continued...