That's a pretty standard cost for gallery rent in high traffic areas in Arizona. To that you have to add insurance, utilities, fees, etc. Basically you are looking at 70k a year or more just to pay for the location and break even. If you have employees you need to add their salary, health care, retirement, etc.
How many prints you need to sell depends on your pricing. The lower the price, the more prints you need to sell and vice versa.
The operational costs assotiated with having a gallery can become a liability very quickly, especially in a receding economy. The rotation of galleries that open and close within a few years (usually 1 or 2 yrs) is quite high.