It is because it is easily pronounceable, and because it is short (4 letters), and these days 4 letter domains with such nice letters and CVCV form are hot.
It's hard to mention an exact figure. I can see it go close to mid $x,xxx because the letters combination is great: two "i" letters, the first syllable is "Hi", sounds good...
It made me think of "Hi girls" but this name is brandable for nearly anything you can think of. Keep it for a while, you'll have a nice sale.