What is an excellent power cable?

Triode Wire Labs. Some of his product comes under your limit.
Also, try thecableco.com. They have a lending program where you can try out some cables, and all you have to pay is return shipping and a 5% restocking fee.

All the best,
Nonoise

I would check out a Magnan. Ken Stevens from CAT was at a demo here in Indy and bought for his entire system

Cullen Cable thinks the Sonarquest makes a difference.  Check out his confusingly named Crossover II PC, now made with the Sonars.

The Cerious Technologies Matrix are a little more than your $500 bogey but will impress the heck out of you.

ozzy

@xenon Many excellent choices within your budget specs.

+1 re. @nonoise ’s recommendation to consider Triode Wire Labs.

You may also want to consider Audio Art Power Cables in addition to the ones other members have already pointed out.

All the best.