All you need for this is a dictionary of zip codes and a rating -- normal, high, very high. Given that ZIPs are 5 digits, that's 100,000 records max, just keep it in memory, you don't even need entries for the "normal" ZIPs. Even if you went street-level, I doubt you'd catalog more than a few hundred thousand streets whose income is significantly more than the surrounding area.

All of this ignores the fact that adjusting a restaurant's prices by the customer's expected ability to pay often leads to killing demand among your most frequent and desirable clientele, but that's a different story.