(MoneyWatch) Rents across the nation are creeping up. But in some cities they're not rising slightly so much as skyrocketing.

Although the average rent across the U.S. is $1,231 per month, in certain areas it can be triple that number. Landlords can afford to charge such daunting prices because there are so few apartments available in high-demand cities.

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"We're experiencing some of the lowest national vacancy rates since the mid-80s," CEO John Kobs said.

The rental vacancy rate has been falling over the past few years. In 2009, the vacancy rate was 12.3 percent, but it fell this past year to 9.3 percent, according to the National Multi Housing Council. And it doesn't look like this trend will change anytime soon.

Buying a home is getting more expensive and more difficult. With mortgage rates and home prices rising, and with mortgage lenders still demanding high down payments and pristine credit scores to get a loan, more potential homebuyers are content to rent.

These factors are combining to push up rental prices, Kobs said. Prices are no steeper than in these 10 cities, where rents can go up to $4,000 per month. calculated the most expensive cities for renters across the U.S. by looking at the 50 most populated cities, which all exceed 350,000 people. The apartment search service then compared the cities' median rental prices for studio, one-bedroom and two-bedroom units.

Of the 10 cities on the list, six are located on the West Coast, while the other four are on the opposite side of the country on the East Coast. Of the six out west, five are located in California, with three of those clustered in the northern end of the state.

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