Hertz Global Holdings, facing a huge pile of debt payments and big bills to upgrade its fleet, is getting the bond market's version of sticker shock.

Refinancing costs are surging after more than a quarter of the car-rental company's market value was obliterated in one week and a gauge of its credit risk jumped to the highest since the financial crisis. Those rising costs could effectively shut Hertz out of the unsecured bond market it has relied on for years to replenish cash and purchase cars. With interest expenses already topping half a billion dollars a year, Hertz may have to pledge more of its assets to creditors to keep those costs from going even higher.

Some of Hertz's bonds are the year's worst performers in Bank of America Merrill Lynch's high-yield services index, with yields hovering near 9%. The junk-rated company needs extra cash to upgrade cars and technology, but it would have to pay hefty premiums — yields of perhaps 10.5% — to get investors to buy any more unsecured debt, said John McClain, a portfolio manager at Diamond Hill Capital Management. At these levels, creditors have to evaluate the company as if they might own it someday, McClain said.