The average monthly rent on a two-bedroom apartment in Boulder reached $2,000 in September, according to the Colorado Rental Price Monitor from Apartment List.

That was the most expensive rent among the 25 cities and areas in Colorado tracked by the San Francisco provider of online apartment listings.

At the other extreme are Clifton and Pueblo, where two-bedroom apartments remain a relative bargain at $600 and $610 a month.

The median one-bedroom rent in the state in September was $1,030, up 6.1 percent from a year ago, and two-bedrooms averaged $1,280, up 6.2 percent.

Apartment List said rents in Colorado are growing at more than twice the annual U.S. average, which is 3 percent.

The Sherrelwood neighborhood in Adams County, just north of U.S. 36 and west of Interstate 25, saw the biggest spikes in the survey, with a 17 percent annual gain to an average of $1,310 for a two-bedroom apartment.

That reflects the ongoing high demand in metro Denver for affordable apartments driving up rents in lower-cost areas.