where is that precisely? SF? LA? what is the income in that zip code?

I have no doubt it's possible in a very high income area where people dont' care about paying $50 for hamburgers, but you're a fool if you think that's generalization to a zip code where the median income is 40-50K.

Also, how long have they been around? months? years?

a few places in my city went that route and closed after 2-3 years. they were experiments.