You can achieve a luxury/expensive/premium aesthetic with some of the following treatments

• Dark background colours
• A single highlight colour: gold, purple or yellow.
• Thin subtle understated illustrative patterns
• High quality typefaces and fine typography, sans or serif, keep it thin though, perhaps use details like ligatures.
• High quality photography of luxurious environments and wealthy looking people.
• Smart, relaxed, confident tone of voice, don't do any hard selling.

If you haven't already, take a look at Uber's brand guide, they achieve this look pretty well. http://brand.uber.com/#menu