I definitely think there's a market for "my", "e" and "i" prefixes for cities. Take a city with a population of only 40,000. There's got to be someone in that city who would plunk down a couple of hundred for one of these names. In fact, if you look at the top 500 cities in America in terms of population, you'll find very few of these prefixes are available, especially for one-word cities.
I think "e" is the most desirable, followed by "i" and then "my".
P.S. I sold an "i" prefix city earlier this week.