some question about Frequency Mixer IC

Hello
Here is a sample of mixer IC's.

1)why is RF output frequency lower than Lo?
2)does it appear sum and difference of LO and IF signal?
3)what parameters increase price of mixer IC's?

If LO frequency is higher than RF frequency then Image Frequency will be higher in frequency and will be easier to be rejected (using filters in front of the mixer).
This is the preferred case in most of heterodyne situations.

f_image = f_RF + 2*f_IF if f_LO > f_RF

f_IF signal in this situation is: f_IF = f_LO - f_RF