What's the most efficient way to market to high-income customers?
Where else do your customers frequent? If they are reading the Robb Report, then an investment there may prove quite profitable. Then again, it's not exactly cheap to advertise with them.
Also, find a PR company to help you get out some publicity for your company. Send out an announcement to some regional or national exposure publications. Perhaps some of the personal finance magazines would entertain having you as an editorial advertiser. This means you get a write-up along with an ad, and sometimes that can be economical.
Ultimately it will still come down to what your budget and marketing plan entails. If you have funding for marketing, then it just needs to be a targeted investment. If you want to do A/B testing, then approach it with smaller budgets and immediately dig into where you get the best response until you find another channel you can find more convertible leads from.