Denver has traditionally been very close to the national average for cost of living, and also has average wages (much lower on both fronts than, say, San Francisco.)

In the last 3 years or so, rents have gone up largely because laborers who can't find work in other cities have been able to find work in or near Denver, driving up demand for housing while simultaneously driving down average wages. This is slowly being alleviated as new housing is being built, but it'll be a few more years before an equilibrium is reached.