Falling mortgage rates and rising home values are a tasty combination for homeowners looking to save some cash on their monthly mortgage payments. Millions, however, are not biting, either out of ignorance or because refinancing is a hassle.

Close to 4.5 million borrowers are eligible and have a financial incentive to refinance, according to a new report from Black Knight Financial Services. That marks an increase of 1.6 million borrowers from just a few months ago, when mortgage rates were higher. The new "refinanceable" population is now at its highest point of the year. The math factors in closing costs.

At today's rates, the average borrower could save $260 a month, or an aggregate savings of $1.1 billion for all of those borrowers. Nearly 700,000 borrowers could save $400 or more per month. To be sure, mortgage applications to refinance have been rising over the past month, up about 9 percent, according to the Mortgage Bankers Association (MBA).

"The recent pause in the upward movement of interest rates continues to encourage late-to-the-game borrowers to refinance," noted Lynn Fisher, MBA's vice president of research and economics.

Refinance applications are still, however, 30 percent below year-ago levels. Millions of borrowers continue to do nothing, making higher monthly payments than they have to.

"Our data doesn't tell us about motivation," said Ben Graboske, senior vice president of data and analytics at BKFS. "It leaves us to surmise that the reason is apathy, lack of awareness and education."

While millions of borrowers have already refinanced at record-low rates, others have been sidelined by various issues. Some are still underwater on their home loans, owing more than the homes are currently worth. Others have had hits to their credit scores since the recession and may not qualify for the lowest rates available.

Those problems, however, are falling away, as home values rise quickly and borrowers rebuild their credit. If rates continue to hold where they are, which is a definite possibility, the refinance population will only increase. Mortgage rates do not directly follow interest rate hikes by the Federal Reserve but are linked to the yield on the 10-year Treasury, which can move on other economic factors both in the U.S. and overseas.

"I don't think this will be the last opportunity, but I don't have a crystal ball," said Graboske. "There are enough pressures in the market — lenders getting more efficient — that we're going to have competitive rates around for a while."